v3.26.1
Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($)
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value [1] $ 50,500,000 $ 24,200,000  
Catastrophe Bonds, Fair Value Disclosure 0.0 1,000,000.0  
Derivative assets 5,000,000.0 17,000,000.0  
Fair Market Value 5,109,300,000 4,978,300,000  
Fair Market Value 1,162,000,000 1,488,700,000  
Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 88,300,000 111,700,000  
Privately-held Investments, at fair value 121,800,000    
Total 6,551,900,000 6,701,700,000  
Recurring | Derivatives at fair value | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative assets 5,000,000.0 17,000,000.0  
Recurring | Liabilities under derivative contracts | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value (12,100,000) (45,900,000)  
Recurring | Liabilities under derivative contracts | Loss Portfolio Transfer      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value   (3,600,000)  
Recurring | Fixed income securities | Non-agency commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 9,500,000    
Recurring | Fixed income securities — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 5,013,400,000 4,692,200,000  
Recurring | Short-term investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 45,400,000 261,900,000  
Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,040,200,000 1,199,900,000  
Catastrophe Bonds, Fair Value Disclosure   1,000,000.0  
Recurring | Real estate fund      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 279,600,000 267,200,000  
Recurring | Privately-held investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value 50,500,000    
Recurring | Equity investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 8,100,000 0  
Recurring | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, at fair value 0    
Total 1,857,900,000 2,207,800,000  
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative assets 0 0  
Recurring | Level 1 | Liabilities under derivative contracts | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value 0 0  
Recurring | Level 1 | Liabilities under derivative contracts | Loss Portfolio Transfer      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value   0  
Recurring | Level 1 | Fixed income securities | Non-agency commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
Recurring | Level 1 | Fixed income securities — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,518,300,000 1,675,700,000  
Recurring | Level 1 | Short-term investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 42,500,000 260,200,000  
Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 289,000,000.0 270,900,000  
Catastrophe Bonds, Fair Value Disclosure   0  
Recurring | Level 1 | Real estate fund      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | Level 1 | Privately-held investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value 0    
Recurring | Level 1 | Equity investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 8,100,000    
Recurring | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, at fair value 0    
Total 4,242,100,000 3,919,300,000  
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative assets 5,000,000.0 17,000,000.0  
Recurring | Level 2 | Liabilities under derivative contracts | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value (12,100,000) (45,900,000)  
Recurring | Level 2 | Liabilities under derivative contracts | Loss Portfolio Transfer      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value   0  
Recurring | Level 2 | Fixed income securities | Non-agency commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 9,500,000    
Recurring | Level 2 | Fixed income securities — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 3,495,100,000 3,016,500,000  
Recurring | Level 2 | Short-term investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,900,000 1,700,000  
Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 751,200,000 929,000,000.0  
Catastrophe Bonds, Fair Value Disclosure   1,000,000.0  
Recurring | Level 2 | Real estate fund      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | Level 2 | Privately-held investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value 0    
Recurring | Level 2 | Equity investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, at fair value 121,800,000    
Total 172,300,000 307,400,000  
Change in unrealized investment gains (losses) 1,400,000    
Fair Market Value 172,300,000    
Recurring | Level 3 | Liability      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities at beginning of year 0 (3,600,000) $ (16,500,000)
Liability purchases 0 0  
Liability transfers 0 0  
Liability, Transfers out of Level 3 0 0  
Liability sales 700,000 0  
Liability, increase (decrease) in fv included in net income 2,900,000 12,900,000  
Change in unrealized gains or losses 0 12,900,000  
Recurring | Level 3 | Loss Portfolio Transfer | Liability      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities at beginning of year 0 (3,600,000) (16,500,000)
Liability purchases 0 0  
Liability transfers 0 0  
Liability, Transfers out of Level 3 0 0  
Liability sales 700,000 0  
Liability, increase (decrease) in fv included in net income 2,900,000 12,900,000  
Change in unrealized gains or losses 0 12,900,000  
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative assets 0 0  
Recurring | Level 3 | Liabilities under derivative contracts | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value 0 0  
Recurring | Level 3 | Liabilities under derivative contracts | Loss Portfolio Transfer      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value   (3,600,000)  
Recurring | Level 3 | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances 126,400,000    
Transferred in or out of Level 3 12,600,000    
Transfers Into Level 3 0    
Settlements and sales (256,000,000.0)    
Increase (decrease) in fair value included in net income 3,500,000    
Fair Market Value   311,000,000.0  
Recurring | Level 3 | Fixed income securities | Non-agency commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
Recurring | Level 3 | Fixed income securities — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | Level 3 | Short-term investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Catastrophe Bonds, Fair Value Disclosure   0  
Recurring | Level 3 | Real estate fund      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Recurring | Level 3 | Commercial mortgage loans      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Change in unrealized investment gains (losses) (300,000) 0  
Fair Market Value 25,400,000 79,700,000  
Recurring | Level 3 | Commercial mortgage loans | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances 100,000 500,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 0 0  
Settlements and sales (56,300,000) (184,400,000)  
Increase (decrease) in fair value included in net income 1,900,000 (11,300,000)  
Fair Market Value   79,700,000 274,900,000
Recurring | Level 3 | Middle market loans and other private debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Change in unrealized investment gains (losses) (2,700,000) 0  
Fair Market Value 28,900,000 61,000,000.0  
Recurring | Level 3 | Middle market loans and other private debt | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances 21,700,000 400,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 0 0  
Settlements and sales (49,400,000) (35,500,000)  
Increase (decrease) in fair value included in net income (4,400,000) 400,000  
Fair Market Value   61,000,000.0 84,800,000
Recurring | Level 3 | Global corporate securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Change in unrealized investment gains (losses) 0 0  
Fair Market Value 0 18,500,000  
Recurring | Level 3 | Global corporate securities | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances 0 4,400,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 0 0  
Settlements and sales (18,600,000) (300,000)  
Increase (decrease) in fair value included in net income 100,000 0  
Fair Market Value   18,500,000 14,400,000
Recurring | Level 3 | Equity Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Change in unrealized investment gains (losses)   0  
Fair Market Value   0  
Recurring | Level 3 | Equity Securities | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances   0  
Transfers Into Level 3   0  
Transfers out of Level 3   0  
Settlements and sales   (7,100,000)  
Increase (decrease) in fair value included in net income   0  
Fair Market Value     18,000,000.0
Recurring | Level 3 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Change in unrealized investment gains (losses)   1,300,000  
Fair Market Value   311,000,000.0  
Recurring | Level 3 | Short-term investments | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances   71,400,000  
Transfers Into Level 3   0  
Transfers out of Level 3   0  
Settlements and sales   (239,700,000)  
Increase (decrease) in fair value included in net income   (10,600,000)  
Fair Market Value     489,900,000
Recurring | Level 3 | Asset-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Change in unrealized investment gains (losses) 3,400,000 1,100,000  
Fair Market Value 67,500,000 127,600,000  
Recurring | Level 3 | Asset-backed | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases and issuances 56,100,000 56,100,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 (2,000,000.0) 0  
Settlements and sales (116,900,000) (12,400,000)  
Increase (decrease) in fair value included in net income 2,700,000 1,000,000.0  
Fair Market Value   127,600,000 82,900,000
Recurring | Level 3 | Privately-held investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value 50,500,000    
Recurring | Level 3 | Equity investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
Recurring | NAV practical expedient      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, at fair value 0 0  
Total 279,600,000 267,200,000  
Recurring | NAV practical expedient | Derivatives at fair value | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative assets 0 0  
Recurring | NAV practical expedient | Liabilities under derivative contracts | Foreign exchange contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value 0 0  
Recurring | NAV practical expedient | Liabilities under derivative contracts | Loss Portfolio Transfer      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities under derivative contracts, fair value   0  
Recurring | NAV practical expedient | Fixed income securities | Non-agency commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | NAV practical expedient | Fixed income securities — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | NAV practical expedient | Short-term investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Catastrophe Bonds, Fair Value Disclosure   0  
Recurring | NAV practical expedient | Short-term investments trading at fair value      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Recurring | NAV practical expedient | Real estate fund      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 279,600,000 267,200,000  
Recurring | NAV practical expedient | Privately-held investments — Available for sale      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value 0 0  
Recurring | NAV practical expedient | Equity investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
U.S. government      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 1,222,000,000 1,480,600,000  
Fair Market Value 263,900,000 261,300,000  
U.S. government | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,222,000,000 1,480,600,000  
U.S. government | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 263,900,000 261,300,000  
U.S. government | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,222,000,000 1,480,600,000  
U.S. government | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 263,900,000 261,300,000  
U.S. government | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. government | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. government | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. government | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. government | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. government | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. agency      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 2,000,000.0 7,200,000  
U.S. agency | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,000,000.0 7,200,000  
U.S. agency | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. agency | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,000,000.0 7,200,000  
U.S. agency | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
U.S. agency | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Municipal      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 92,300,000 82,300,000  
Fair Market Value 4,700,000 1,600,000  
Municipal | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 92,300,000 82,300,000  
Municipal | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 4,700,000 1,600,000  
Municipal | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Municipal | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Municipal | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 92,300,000 82,300,000  
Municipal | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 4,700,000 1,600,000  
Municipal | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Municipal | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Municipal | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Municipal | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Corporate      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 1,997,600,000 1,986,400,000  
Fair Market Value 141,200,000 151,100,000  
Corporate | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,997,600,000 1,986,400,000  
Corporate | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 141,200,000 151,100,000  
Corporate | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Corporate | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Corporate | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,997,600,000 1,986,400,000  
Corporate | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 141,200,000 151,100,000  
Corporate | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Corporate | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Corporate | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Corporate | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government-backed corporate      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 66,300,000 131,300,000  
Fair Market Value   2,800,000  
Non-U.S. government-backed corporate | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 66,300,000 131,300,000  
Non-U.S. government-backed corporate | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   2,800,000  
Non-U.S. government-backed corporate | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government-backed corporate | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Non-U.S. government-backed corporate | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 66,300,000 131,300,000  
Non-U.S. government-backed corporate | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   2,800,000  
Non-U.S. government-backed corporate | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government-backed corporate | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Non-U.S. government-backed corporate | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government-backed corporate | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Non-U.S. government      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 401,600,000 246,800,000  
Fair Market Value 37,600,000 24,400,000  
Non-U.S. government | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 401,600,000 246,800,000  
Non-U.S. government | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 37,600,000 24,400,000  
Non-U.S. government | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 296,300,000 195,100,000  
Non-U.S. government | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 25,100,000 9,600,000  
Non-U.S. government | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 105,300,000 51,700,000  
Non-U.S. government | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 12,500,000 14,800,000  
Non-U.S. government | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-U.S. government | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Asset-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 501,000,000.0 234,500,000  
Fair Market Value 460,700,000 625,200,000  
Asset-backed | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 501,000,000.0 234,500,000  
Asset-backed | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 460,700,000 625,200,000  
Asset-backed | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Asset-backed | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Asset-backed | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 501,000,000.0 234,500,000  
Asset-backed | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 460,700,000 625,200,000  
Asset-backed | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Asset-backed | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Asset-backed | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Asset-backed | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Non-agency commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 9,500,000 4,400,000  
Non-agency commercial mortgage-backed securities | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   4,400,000  
Non-agency commercial mortgage-backed securities | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Non-agency commercial mortgage-backed securities | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   4,400,000  
Non-agency commercial mortgage-backed securities | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Agency residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 721,100,000 518,700,000  
Agency residential mortgage-backed securities | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 721,100,000    
Agency residential mortgage-backed securities | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
Agency residential mortgage-backed securities | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 721,100,000    
Agency residential mortgage-backed securities | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0    
Agency residential mortgage-backed securities | Recurring | NAV practical expedient | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Agency mortgage-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 32,100,000 31,100,000  
Agency mortgage-backed | Recurring | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   518,700,000  
Agency mortgage-backed | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 32,100,000 31,100,000  
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Agency mortgage-backed | Recurring | Level 2 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   518,700,000  
Agency mortgage-backed | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 32,100,000 31,100,000  
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Agency mortgage-backed | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
High yield loans      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 100,000,000.0 102,400,000  
High yield loans | Recurring | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 100,000,000.0 102,400,000  
High yield loans | Recurring | Level 1 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
High yield loans | Recurring | Level 2 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 100,000,000.0 102,400,000  
High yield loans | Recurring | Level 3 | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
High yield loans | Recurring | NAV practical expedient | Fixed income securities, trading      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0  
Fixed income securities, trading | Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value   24,200,000  
Privately-held Investments, at fair value   286,800,000  
Fixed income securities, trading | Recurring | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value   0  
Privately-held Investments, at fair value   0  
Fixed income securities, trading | Recurring | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value   0  
Privately-held Investments, at fair value   0  
Fixed income securities, trading | Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Privately-held Investments, available for sale, at fair value   24,200,000  
Privately-held Investments, at fair value   286,800,000  
Short-term investments trading at fair value | Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   1,000,000.0  
Short-term investments trading at fair value | Recurring | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   1,000,000.0  
Short-term investments trading at fair value | Recurring | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Short-term investments trading at fair value | Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value   0  
Asset backed securities, privately-held afs      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 16,300,000 24,200,000  
Asset backed securities, privately-held afs | Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value 16,300,000 24,200,000  
Change in unrealized investment gains (losses) 600,000 200,000  
Asset backed securities, privately-held afs | Recurring | Level 3 | Assets      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Market Value   24,200,000 $ 14,900,000
Purchases and issuances 14,700,000 10,000,000.0  
Transfers Into Level 3 0 0  
Transfers out of Level 3 (10,600,000) 0  
Settlements and sales (14,800,000) 0  
Increase (decrease) in fair value included in net income $ 2,800,000 $ (700,000)  
[1] Privately-held investments, trading at fair value include related party investments totaling $4.8 million (2024 — $73.6 million).