v3.26.1
Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details)
Dec. 31, 2025
USD ($)
Integer
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Claims Development [Line Items]                    
Liability for claims and claim adjustment expenses, net of reinsurance $ 4,492,400,000 $ 4,116,600,000                
Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,683,300,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,472,200,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 211,100,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 211,100,000 268,900,000                
Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,595,400,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 697,100,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 898,300,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 898,300,000 756,000,000.0                
Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,266,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,046,000,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 220,100,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 220,100,000 221,300,000                
Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,662,700,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,553,600,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 1,109,100,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 1,109,100,000 981,300,000                
Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 3,035,600,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 2,548,300,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 487,300,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 487,300,000 560,600,000                
Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,946,700,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 882,700,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 1,064,000,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 1,064,000,000 885,900,000                
Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,481,300,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,978,800,000                  
All outstanding liabilities for 2016 and subsequent years, net of reinsurance 502,500,000                  
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 502,500,000 442,600,000                
Accident Year 2016 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 235,900,000 235,900,000 $ 235,900,000 $ 235,900,000 $ 236,300,000 $ 248,000,000.0 $ 246,600,000 $ 245,200,000 $ 250,400,000 $ 239,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 10,758                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 235,900,000 235,900,000 235,900,000 235,900,000 236,300,000 233,500,000 224,500,000 202,200,000 169,500,000 67,200,000
Accident Year 2016 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 127,200,000 127,200,000 127,200,000 127,200,000 248,000,000.0 202,900,000 191,700,000 185,100,000 189,700,000 218,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,886                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 127,200,000 127,200,000 127,200,000 127,200,000 126,200,000 110,800,000 83,400,000 40,600,000 23,100,000 4,200,000
Accident Year 2016 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 197,300,000 197,300,000 197,300,000 197,300,000 221,900,000 220,200,000 230,400,000 229,800,000 230,000,000.0 260,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,443                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 197,300,000 197,300,000 197,300,000 197,300,000 194,900,000 192,200,000 165,100,000 143,500,000 83,000,000.0 30,900,000
Accident Year 2016 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 135,300,000 135,300,000 135,300,000 135,300,000 187,500,000 186,300,000 202,600,000 217,000,000.0 212,400,000 191,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,250                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 135,300,000 135,300,000 135,300,000 135,300,000 131,100,000 126,500,000 130,400,000 102,400,000 71,500,000 15,300,000
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 228,300,000 228,300,000 228,300,000 228,300,000 229,900,000 238,200,000 242,900,000 263,900,000 265,300,000 266,000,000.0
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,287                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 228,300,000 228,300,000 228,300,000 228,300,000 229,900,000 225,800,000 212,700,000 202,000,000.0 161,500,000 55,900,000
Accident Year 2016 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 138,200,000 138,200,000 138,200,000 138,200,000 253,100,000 262,400,000 255,100,000 244,700,000 245,200,000 232,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,317                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 138,200,000 138,200,000 138,200,000 138,200,000 138,300,000 126,500,000 96,400,000 64,300,000 33,600,000 9,200,000
Accident Year 2016 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 223,700,000 223,700,000 223,700,000 223,700,000 213,700,000 226,000,000.0 231,100,000 238,600,000 240,500,000 239,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 940                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 223,700,000 223,700,000 223,700,000 223,700,000 196,200,000 194,800,000 184,500,000 166,500,000 151,600,000 $ 58,700,000
Accident Year 2017 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 185,600,000 185,600,000 185,600,000 185,600,000 275,500,000 254,600,000 253,200,000 260,100,000 297,200,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 9,719                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 185,600,000 185,600,000 185,600,000 185,600,000 237,800,000 243,900,000 223,000,000.0 189,700,000 97,000,000.0  
Accident Year 2017 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 72,700,000 72,700,000 72,700,000 72,700,000 221,100,000 199,000,000.0 180,100,000 176,000,000.0 182,600,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,585                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 72,700,000 72,700,000 72,700,000 72,700,000 92,400,000 98,200,000 53,300,000 23,100,000 3,600,000  
Accident Year 2017 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 127,000,000.0 127,000,000.0 127,000,000.0 127,000,000.0 226,100,000 214,800,000 207,300,000 200,500,000 210,000,000.0  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,080                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 127,000,000.0 127,000,000.0 127,000,000.0 127,000,000.0 150,500,000 168,800,000 140,700,000 97,900,000 40,300,000  
Accident Year 2017 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 137,400,000 137,400,000 137,400,000 137,400,000 210,900,000 188,900,000 188,100,000 183,300,000 206,900,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,766                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 137,400,000 137,400,000 137,400,000 137,400,000 135,900,000 117,400,000 83,400,000 51,200,000 27,200,000  
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 437,500,000 437,500,000 437,500,000 437,500,000 574,600,000 502,900,000 514,100,000 532,700,000 554,000,000.0  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,948                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 437,500,000 437,500,000 437,500,000 437,500,000 412,500,000 439,300,000 415,100,000 356,400,000 123,300,000  
Accident Year 2017 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 111,500,000 111,500,000 111,500,000 111,500,000 263,600,000 252,200,000 252,800,000 241,900,000 244,400,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,431                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 111,500,000 111,500,000 111,500,000 111,500,000 111,400,000 97,500,000 59,200,000 30,600,000 8,900,000  
Accident Year 2017 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 364,000,000.0 364,000,000.0 364,000,000.0 364,000,000.0 359,100,000 365,600,000 377,100,000 393,400,000 380,300,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,344                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 364,000,000.0 364,000,000.0 364,000,000.0 364,000,000.0 308,700,000 307,400,000 271,800,000 239,600,000 $ 94,900,000  
Accident Year 2018 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 190,200,000 190,200,000 190,200,000 190,200,000 199,800,000 191,700,000 208,100,000 205,400,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 8,201                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 190,200,000 190,200,000 190,200,000 190,200,000 178,600,000 184,900,000 162,800,000 62,600,000    
Accident Year 2018 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 45,600,000 45,600,000 45,600,000 45,600,000 169,000,000.0 137,700,000 126,600,000 123,700,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,551                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 45,600,000 45,600,000 45,600,000 45,600,000 59,900,000 44,000,000.0 28,100,000 3,200,000    
Accident Year 2018 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 151,500,000 151,500,000 151,500,000 151,500,000 235,100,000 208,900,000 207,800,000 170,800,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,213                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 151,500,000 151,500,000 151,500,000 151,500,000 151,500,000 133,400,000 105,100,000 26,800,000    
Accident Year 2018 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 112,700,000 112,700,000 112,700,000 112,700,000 163,600,000 154,900,000 173,300,000 157,500,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,661                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 112,700,000 112,700,000 112,700,000 112,700,000 112,300,000 100,100,000 75,000,000.0 20,500,000    
Accident Year 2018 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 288,600,000 288,600,000 288,600,000 288,600,000 518,100,000 326,900,000 336,000,000.0 309,800,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,809                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 288,600,000 288,600,000 288,600,000 288,600,000 263,300,000 270,500,000 270,400,000 122,500,000    
Accident Year 2018 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 93,100,000 93,100,000 93,100,000 93,100,000 255,800,000 265,700,000 258,000,000.0 228,400,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,372                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 93,100,000 93,100,000 93,100,000 93,100,000 93,100,000 73,600,000 33,600,000 7,100,000    
Accident Year 2018 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 328,300,000 328,300,000 328,300,000 328,300,000 418,200,000 393,500,000 395,300,000 395,000,000.0    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,437                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 328,300,000 328,300,000 328,300,000 328,300,000 326,200,000 314,400,000 280,600,000 $ 27,600,000    
Accident Year 2019 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 190,800,000 190,800,000 190,800,000 190,800,000 104,300,000 131,900,000 128,200,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,875                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 190,800,000 190,800,000 190,800,000 190,800,000 100,300,000 92,400,000 49,500,000      
Accident Year 2019 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 52,600,000 52,600,000 52,600,000 52,600,000 156,800,000 148,600,000 126,200,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,269                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 52,600,000 52,600,000 52,600,000 52,600,000 65,300,000 18,500,000 6,400,000      
Accident Year 2019 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 107,600,000 107,600,000 107,600,000 107,600,000 124,000,000.0 153,000,000.0 145,600,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 3,709                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 107,600,000 107,600,000 107,600,000 107,600,000 89,800,000 72,600,000 33,600,000      
Accident Year 2019 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 132,000,000.0 132,000,000.0 132,000,000.0 132,000,000.0 239,500,000 261,900,000 249,300,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 23,804                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 132,000,000.0 132,000,000.0 132,000,000.0 132,000,000.0 121,700,000 87,000,000.0 27,300,000      
Accident Year 2019 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 160,500,000 160,500,000 160,500,000 160,500,000 321,200,000 230,400,000 214,700,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,410                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 160,500,000 160,500,000 160,500,000 160,500,000 161,700,000 136,300,000 28,300,000      
Accident Year 2019 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 53,000,000.0 53,000,000.0 53,000,000.0 53,000,000.0 245,000,000.0 254,700,000 234,400,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,056                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 53,000,000.0 53,000,000.0 53,000,000.0 53,000,000.0 52,600,000 36,400,000 9,300,000      
Accident Year 2019 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 400,900,000 400,900,000 400,900,000 400,900,000 401,200,000 498,300,000 474,800,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,563                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 400,900,000 400,900,000 400,900,000 400,900,000 401,000,000.0 382,500,000 $ 274,000,000.0      
Accident Year 2020 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 205,700,000 216,900,000 216,500,000 210,800,000 200,700,000 206,400,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 8,500,000                  
Number of Reported Claims | Integer 7,689                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 188,700,000 180,600,000 171,500,000 153,400,000 125,300,000 61,800,000        
Accident Year 2020 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 155,200,000 154,000,000.0 147,900,000 142,100,000 143,800,000 134,600,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 33,200,000                  
Number of Reported Claims | Integer 3,905                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 102,900,000 92,700,000 62,000,000.0 36,500,000 9,500,000 0        
Accident Year 2020 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 145,700,000 151,700,000 126,800,000 125,800,000 111,300,000 110,300,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 26,300,000                  
Number of Reported Claims | Integer 3,874                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 114,200,000 109,700,000 101,300,000 88,800,000 66,600,000 28,600,000        
Accident Year 2020 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 374,400,000 355,900,000 353,900,000 339,700,000 349,400,000 349,600,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 63,500,000                  
Number of Reported Claims | Integer 106,017                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 293,500,000 241,600,000 227,000,000.0 175,100,000 121,700,000 47,900,000        
Accident Year 2020 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 376,700,000 377,900,000 365,300,000 354,300,000 406,100,000 318,500,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 5,600,000                  
Number of Reported Claims | Integer 1,464                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 349,200,000 312,600,000 312,600,000 237,100,000 165,500,000 41,800,000        
Accident Year 2020 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 206,600,000 190,800,000 181,600,000 199,700,000 235,900,000 254,600,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 37,500,000                  
Number of Reported Claims | Integer 1,723                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 126,800,000 102,400,000 71,900,000 44,400,000 27,900,000 9,200,000        
Accident Year 2020 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 374,700,000 372,700,000 380,300,000 376,500,000 602,300,000 415,000,000.0        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 12,600,000                  
Number of Reported Claims | Integer 1,538                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 344,300,000 335,100,000 312,100,000 291,400,000 270,500,000 $ 213,300,000        
Accident Year 2021 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 190,800,000 194,400,000 197,900,000 204,700,000 210,400,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 5,600,000                  
Number of Reported Claims | Integer 6,828                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 179,500,000 165,300,000 152,700,000 120,800,000 59,200,000          
Accident Year 2021 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 191,900,000 198,200,000 200,300,000 191,600,000 176,500,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 56,900,000                  
Number of Reported Claims | Integer 3,687                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 107,600,000 92,700,000 54,300,000 23,700,000 3,200,000          
Accident Year 2021 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 94,700,000 97,200,000 95,400,000 95,800,000 93,000,000.0          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 10,100,000                  
Number of Reported Claims | Integer 4,785                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 82,300,000 70,600,000 64,600,000 52,200,000 23,500,000          
Accident Year 2021 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 284,800,000 297,900,000 296,800,000 306,500,000 287,800,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 46,900,000                  
Number of Reported Claims | Integer 34,955                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 198,900,000 162,800,000 131,300,000 89,900,000 43,600,000          
Accident Year 2021 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 496,200,000 504,500,000 512,100,000 492,400,000 663,100,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,100,000                  
Number of Reported Claims | Integer 1,530                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 467,000,000.0 442,500,000 375,000,000.0 241,600,000 75,500,000          
Accident Year 2021 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 207,500,000 202,000,000.0 206,300,000 217,100,000 207,200,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 52,100,000                  
Number of Reported Claims | Integer 1,927                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 123,000,000.0 97,500,000 64,100,000 37,500,000 7,900,000          
Accident Year 2021 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 124,400,000 138,500,000 142,300,000 153,000,000.0 157,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 8,700,000                  
Number of Reported Claims | Integer 1,404                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 98,900,000 89,100,000 76,200,000 53,500,000 $ 28,500,000          
Accident Year 2022 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 176,000,000.0 174,000,000.0 177,300,000 170,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 8,000,000.0                  
Number of Reported Claims | Integer 5,884                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 159,900,000 145,800,000 115,200,000 41,500,000            
Accident Year 2022 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 216,200,000 214,100,000 218,500,000 207,900,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 97,600,000                  
Number of Reported Claims | Integer 3,628                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 79,100,000 47,500,000 27,200,000 9,000,000.0            
Accident Year 2022 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 105,900,000 113,500,000 106,400,000 108,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 18,200,000                  
Number of Reported Claims | Integer 5,405                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 81,500,000 73,900,000 57,700,000 24,900,000            
Accident Year 2022 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 279,600,000 272,800,000 304,300,000 319,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 90,200,000                  
Number of Reported Claims | Integer 3,339                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 154,800,000 114,900,000 76,000,000.0 18,000,000.0            
Accident Year 2022 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 386,000,000.0 387,800,000 394,200,000 395,900,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 42,100,000                  
Number of Reported Claims | Integer 1,505                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 305,100,000 271,800,000 201,300,000 65,200,000            
Accident Year 2022 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 249,400,000 250,000,000.0 251,600,000 250,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 92,100,000                  
Number of Reported Claims | Integer 2,234                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 104,200,000 60,500,000 31,300,000 9,500,000            
Accident Year 2022 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 251,600,000 200,100,000 195,300,000 195,400,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 130,000,000.0                  
Number of Reported Claims | Integer 1,551                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 98,600,000 76,100,000 54,400,000 $ 26,300,000            
Accident Year 2023 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 102,800,000 135,400,000 156,300,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 10,900,000                  
Number of Reported Claims | Integer 4,206                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 72,600,000 67,300,000 30,400,000              
Accident Year 2023 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 251,100,000 239,200,000 228,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 106,600,000                  
Number of Reported Claims | Integer 3,197                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 85,700,000 36,600,000 5,600,000              
Accident Year 2023 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 105,400,000 110,800,000 117,300,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 12,200,000                  
Number of Reported Claims | Integer 4,073                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 86,000,000.0 73,400,000 27,800,000              
Accident Year 2023 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 350,700,000 351,700,000 343,400,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 119,300,000                  
Number of Reported Claims | Integer 3,292                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 166,200,000 101,100,000 21,600,000              
Accident Year 2023 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 202,600,000 225,500,000 230,700,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 16,800,000                  
Number of Reported Claims | Integer 1,153                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 154,200,000 107,300,000 44,800,000              
Accident Year 2023 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 271,600,000 272,200,000 272,900,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 137,400,000                  
Number of Reported Claims | Integer 2,393                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 82,000,000.0 35,600,000 8,500,000              
Accident Year 2023 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 124,300,000 146,000,000.0 143,200,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 36,900,000                  
Number of Reported Claims | Integer 1,324                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 68,800,000 48,800,000 $ 22,700,000              
Accident Year 2024 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 95,900,000 138,300,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 37,600,000                  
Number of Reported Claims 2,494                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 43,300,000 19,600,000                
Accident Year 2024 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 243,200,000 240,500,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 185,300,000                  
Number of Reported Claims 2,423                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 20,700,000 3,400,000                
Accident Year 2024 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 102,800,000 110,000,000.0                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 27,900,000                  
Number of Reported Claims 3,661                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 59,500,000 32,400,000                
Accident Year 2024 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 436,600,000 428,800,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 203,000,000.0                  
Number of Reported Claims 4,469                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 168,200,000 90,400,000                
Accident Year 2024 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 237,000,000.0 262,600,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 51,400,000                  
Number of Reported Claims | Integer 1,053                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 112,200,000 33,300,000                
Accident Year 2024 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 298,700,000 291,900,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 220,400,000                  
Number of Reported Claims | Integer 1,996                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 38,800,000 9,700,000                
Accident Year 2024 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 152,400,000 155,500,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 99,300,000                  
Number of Reported Claims | Integer 1,081                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 34,200,000 $ 14,400,000                
Accident Year 2025 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 109,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 31,200,000                  
Number of Reported Claims 1,781                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 25,700,000                  
Accident Year 2025 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 239,700,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 212,400,000                  
Number of Reported Claims 1,767                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 3,000,000.0                  
Accident Year 2025 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 128,200,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 66,800,000                  
Number of Reported Claims 2,351                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 39,100,000                  
Accident Year 2025 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 419,200,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 332,800,000                  
Number of Reported Claims 4,730                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 54,600,000                  
Accident Year 2025 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 222,200,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 99,200,000                  
Number of Reported Claims | Integer 702                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 45,700,000                  
Accident Year 2025 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 317,100,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 286,900,000                  
Number of Reported Claims | Integer 1,024                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 12,100,000                  
Accident Year 2025 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 137,000,000.0                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 106,200,000                  
Number of Reported Claims | Integer 833                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 17,100,000