v3.26.1
Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period $ 435,600,000 $ 376,100,000 $ 343,600,000 $ 209,000,000.0
Included in Condensed Consolidated Statement of Operations (100,000) 2,200,000 (1,600,000) 3,400,000
Included in Other Comprehensive Income (Loss) 2,600,000 100,000 700,000 2,600,000
Purchases 152,400,000 87,400,000 287,400,000 257,200,000
Sales (100,600,000) (61,000,000.0) (130,800,000) (82,300,000)
Transfers into Level 3 7,200,000 0 15,400,000 15,000,000.0
Transfers out of Level 3 (11,400,000) (58,600,000) (29,000,000.0) (58,700,000)
Balance, End of Period 485,700,000 346,200,000 485,700,000 346,200,000
Corporate Bonds and Notes        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 379,600,000 275,800,000 293,000,000.0 194,200,000
Included in Condensed Consolidated Statement of Operations 200,000 300,000 400,000 1,500,000
Included in Other Comprehensive Income (Loss) 1,800,000 900,000 (1,100,000) 2,700,000
Purchases 132,000,000.0 33,900,000 242,700,000 128,900,000
Sales (94,500,000) (49,200,000) (122,600,000) (70,500,000)
Transfers into Level 3 6,900,000 0 13,600,000 5,000,000.0
Transfers out of Level 3 0 (100,000) 0 (200,000)
Balance, End of Period 426,000,000.0 261,600,000 426,000,000.0 261,600,000
Foreign Governments        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 6,100,000   0  
Included in Condensed Consolidated Statement of Operations 0   0  
Included in Other Comprehensive Income (Loss) 0   0  
Purchases 0   6,100,000  
Sales (6,100,000)   (6,100,000)  
Transfers into Level 3 0   0  
Transfers out of Level 3 0   0  
Balance, End of Period 0   0  
States and Political Subdivisions        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 1,600,000 1,700,000 1,700,000 1,800,000
Included in Condensed Consolidated Statement of Operations 0 0 0 0
Included in Other Comprehensive Income (Loss) 0 0 (100,000) (100,000)
Purchases 0 0 0 0
Sales 0 0 0 0
Transfers into Level 3 0 0 0 0
Transfers out of Level 3 (1,400,000) (1,500,000) (1,400,000) (1,500,000)
Balance, End of Period 200,000 200,000 200,000 200,000
Redeemable Preferred Stocks        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 4,000,000.0 4,300,000 4,400,000 4,200,000
Included in Condensed Consolidated Statement of Operations 0 0 0 0
Included in Other Comprehensive Income (Loss) 0 0 100,000 100,000
Purchases 0 0 0 0
Sales 0 0 (500,000) 0
Transfers into Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Balance, End of Period 4,000,000.0 4,300,000 4,000,000.0 4,300,000
Collateralized Loan Obligations        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 10,000,000.0 67,000,000.0 12,400,000 0
Included in Condensed Consolidated Statement of Operations 0 0 (1,400,000) 0
Included in Other Comprehensive Income (Loss) 0 0 1,500,000 0
Purchases 4,000,000.0 52,800,000 14,000,000.0 119,800,000
Sales 0 (10,000,000.0) (1,600,000) (10,000,000.0)
Transfers into Level 3 0 0 1,500,000 0
Transfers out of Level 3 (10,000,000.0) (57,000,000.0) (22,400,000) (57,000,000.0)
Balance, End of Period 4,000,000.0 52,800,000 4,000,000.0 52,800,000
Other Mortgage- and Asset-backed        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 30,000,000.0 22,500,000 27,900,000 5,000,000.0
Included in Condensed Consolidated Statement of Operations (400,000) 0 (700,000) 0
Included in Other Comprehensive Income (Loss) 0 (100,000) (600,000) (100,000)
Purchases 16,400,000 200,000 24,600,000 7,700,000
Sales 0 (100,000) 0 (100,000)
Transfers into Level 3 0 0 0 10,000,000.0
Transfers out of Level 3 0 0 (5,200,000) 0
Balance, End of Period 46,000,000.0 22,500,000 46,000,000.0 22,500,000
Equity Securities        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, Beginning of Period 4,300,000 4,800,000 4,200,000 3,800,000
Included in Condensed Consolidated Statement of Operations 100,000 1,900,000 100,000 1,900,000
Included in Other Comprehensive Income (Loss) 800,000 (700,000) 900,000 0
Purchases 0 500,000 0 800,000
Sales 0 (1,700,000) 0 (1,700,000)
Transfers into Level 3 300,000 0 300,000 0
Transfers out of Level 3 0 0 0 0
Balance, End of Period $ 5,500,000 $ 4,800,000 $ 5,500,000 $ 4,800,000