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Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Accumulated other comprehensive (loss)    
Fair Market Value $ 4,978.3 $ 4,231.1  
Fair Market Value 1,488.7 1,964.4  
Level 3 | Fair Value, Recurring [Member]      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) 1.3 (15.7)  
Fair Market Value 311.0 489.9  
Level 3 | Fair Value, Recurring [Member] | Liability      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Asset reclassification 0.0 0.0  
Liabilities at beginning of year (3.6) (16.5) $ (31.7)
Liability purchases 0.0 0.0  
Liability transfers 0.0 0.0  
Liability, Transfers out of Level 3 0.0 0.0  
Liability sales 0.0 0.0  
Liability, increase (decrease) in fv included in net income 12.9 15.2  
Change in unrealized gains or losses 12.9 15.2  
Level 3 | Fair Value, Recurring [Member] | Loss Portfolio Transfer | Liability      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Asset reclassification 0.0 0.0  
Liabilities at beginning of year (3.6) (16.5) (31.7)
Liability purchases 0.0 0.0  
Liability transfers 0.0 0.0  
Liability, Transfers out of Level 3 0.0 0.0  
Liability sales 0.0 0.0  
Liability, increase (decrease) in fv included in net income 12.9 15.2  
Change in unrealized gains or losses 12.9 15.2  
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) 0.0 (17.9)  
Fair Market Value 79.7 274.9  
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) 0.0 0.5  
Fair Market Value 61.0 84.8  
Level 3 | Fair Value, Recurring [Member] | Asset-backed      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses) 1.1 1.7  
Fair Market Value 127.6 82.9  
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 (0.2)  
Fair Market Value 18.5 14.4  
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) 0.0 0.0  
Fair Market Value 0.0 18.0  
Level 3 | Fair Value, Recurring [Member] | Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Change in unrealized investment gains (losses)   0.0  
Fair Market Value   0.0  
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 24.2 14.9  
Change in unrealized investment gains (losses) 0.2 0.2  
Assets | Level 3 | Fair Value, Recurring [Member]      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 71.4 113.7  
Transfers Into Level 3 0.0 5.3  
Transfers out of Level 3 0.0 12.1  
Asset reclassification 0.0 0.0  
Settlements and sales (239.7) (137.2)  
Increase (decrease) in fair value included in net income (10.6) (12.8)  
Fair Market Value   489.9 533.0
Assets | Level 3 | Fair Value, Recurring [Member] | Commercial mortgage loans      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 0.5 40.6  
Transfers Into Level 3 0.0 0.0  
Transfers out of Level 3 0.0 0.0  
Asset reclassification 0.0 0.0  
Settlements and sales (184.4) (61.5)  
Increase (decrease) in fair value included in net income (11.3) (16.3)  
Fair Market Value   274.9 312.1
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 0.4 18.3  
Transfers Into Level 3 0.0 0.0  
Transfers out of Level 3 0.0 0.0  
Asset reclassification 10.9 0.0  
Settlements and sales (35.5) (41.9)  
Increase (decrease) in fair value included in net income 0.4 1.5  
Fair Market Value   84.8 106.9
Assets | Level 3 | Fair Value, Recurring [Member] | Asset-backed      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 56.1 21.9  
Transfers Into Level 3 0.0 5.3  
Transfers out of Level 3 0.0 (5.5)  
Asset reclassification 0.0 0.0  
Settlements and sales (12.4) (7.6)  
Increase (decrease) in fair value included in net income 1.0 2.0  
Fair Market Value   82.9 66.8
Assets | Level 3 | Fair Value, Recurring [Member] | Global corporate securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 4.4 0.0  
Transfers Into Level 3 0.0 0.0  
Transfers out of Level 3 0.0 0.0  
Asset reclassification 0.0 0.0  
Settlements and sales (0.3) (0.4)  
Increase (decrease) in fair value included in net income 0.0 (0.2)  
Fair Market Value   14.4 15.0
Assets | Level 3 | Fair Value, Recurring [Member] | Short-term investments      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances 0.0 18.2  
Transfers Into Level 3 0.0 0.0  
Transfers out of Level 3 0.0 0.0  
Asset reclassification (10.9) 0.0  
Settlements and sales (7.1) (25.8)  
Increase (decrease) in fair value included in net income 0.0 0.0  
Fair Market Value   18.0 25.6
Assets | Level 3 | Fair Value, Recurring [Member] | Equity Securities      
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]      
Purchases and issuances   0.0  
Transfers Into Level 3   0.0  
Transfers out of Level 3   (6.6)  
Asset reclassification   0.0  
Settlements and sales   0.0  
Increase (decrease) in fair value included in net income   0.0  
Fair Market Value     6.6
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   14.9 $ 0.0
Purchases and issuances 10.0 14.7  
Transfers Into Level 3 0.0 0.0  
Transfers out of Level 3 0.0 0.0  
Asset reclassification 0.0 0.0  
Settlements and sales 0.0 0.0  
Increase (decrease) in fair value included in net income $ (0.7) $ 0.2