v3.26.1
Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($)
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Market Value $ 5,109,300,000 $ 4,978,300,000  
Fair Market Value 1,162,000,000 1,488,700,000  
Asset backed securities, privately-held afs      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Market Value 16,300,000 24,200,000  
Privately-held investments, global corporate securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Market Value 34,200,000    
Level 3 | Fair Value, Recurring [Member]      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) 1,400,000    
Fair Market Value $ 172,300,000    
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Change in net unrealized gains/(losses) on available for sale securities held    
Level 3 | Fair Value, Recurring [Member] | Liability      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Asset reclassification $ 0 0  
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  
Level 3 | Fair Value, Recurring [Member] | Loss Portfolio Transfer | Liability      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Asset reclassification 0 0  
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  
Level 3 | Fair Value, Recurring [Member] | Commercial mortgage loans      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) (300,000) 0  
Fair Market Value 25,400,000 79,700,000  
Level 3 | Fair Value, Recurring [Member] | Middle market loans and other private debt      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) (2,700,000) 0  
Fair Market Value 28,900,000 61,000,000.0  
Level 3 | Fair Value, Recurring [Member] | Asset-backed      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) 3,400,000 1,100,000  
Fair Market Value 67,500,000 127,600,000  
Level 3 | Fair Value, Recurring [Member] | Global corporate securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) 0 0  
Fair Market Value 0 18,500,000  
Level 3 | Fair Value, Recurring [Member] | Short-term investments      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses)   1,300,000  
Fair Market Value   311,000,000.0  
Level 3 | Fair Value, Recurring [Member] | Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses)   0  
Fair Market Value   0  
Level 3 | Fair Value, Recurring [Member] | Asset backed securities, privately-held afs      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Market Value 16,300,000 24,200,000  
Change in unrealized investment gains (losses) 600,000 200,000  
Level 3 | Fair Value, Recurring [Member] | Privately-held investments, global corporate securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Market Value 34,200,000    
Change in unrealized investment gains (losses) 400,000    
Assets | Level 3 | Fair Value, Recurring [Member]      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 126,400,000    
Transfers Into Level 3 0    
Asset reclassification 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  
Assets | Level 3 | Fair Value, Recurring [Member] | Commercial mortgage loans      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 100,000 500,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 0 0  
Asset reclassification 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
Assets | Level 3 | Fair Value, Recurring [Member] | Middle market loans and other private debt      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 21,700,000 400,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 0 0  
Asset reclassification 0 10,900,000  
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
Assets | Level 3 | Fair Value, Recurring [Member] | Asset-backed      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
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  
Asset reclassification 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
Assets | Level 3 | Fair Value, Recurring [Member] | Global corporate securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 0 4,400,000  
Transfers Into Level 3 0 0  
Transfers out of Level 3 0 0  
Asset reclassification 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
Assets | Level 3 | Fair Value, Recurring [Member] | Short-term investments      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances   71,400,000  
Transfers Into Level 3   0  
Transfers out of Level 3   0  
Asset reclassification   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
Assets | Level 3 | Fair Value, Recurring [Member] | Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances   0  
Transfers Into Level 3   0  
Transfers out of Level 3   0  
Asset reclassification   (10,900,000)  
Settlements and sales   (7,100,000)  
Increase (decrease) in fair value included in net income   0  
Fair Market Value     18,000,000.0
Assets | Level 3 | Fair Value, Recurring [Member] | Asset backed securities, privately-held afs      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
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  
Asset reclassification 0 0  
Settlements and sales (14,800,000) 0  
Increase (decrease) in fair value included in net income 2,800,000 (700,000)  
Assets | Level 3 | Fair Value, Recurring [Member] | Privately-held investments, global corporate securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Market Value   $ 0  
Purchases and issuances 33,800,000    
Transfers Into Level 3 0    
Transfers out of Level 3 0    
Asset reclassification 0    
Settlements and sales 0    
Increase (decrease) in fair value included in net income $ 400,000