v3.25.1
Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2025
Dec. 31, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 156.6 $ 56.8
Transfers into Level 3 18.4 0.7
Transfers out of Level 3 (0.7) (10.1)
Total gains or losses (realized/unrealized):    
Included in net income 1.0 29.8
Included in other comprehensive income 1.9 (0.7)
Purchases, issuances, sales, and settlements:    
Purchases 11.3 91.7
Sales (0.5) (1.7)
Settlements 0.0 (9.9)
Ending balance 188.0 156.6
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period (2.8) (28.4)
Fixed maturities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 36.6 50.4
Transfers into Level 3 14.1 0.7
Transfers out of Level 3 (0.7) (10.1)
Total gains or losses (realized/unrealized):    
Included in net income (1.0) 1.5
Included in other comprehensive income 1.9 (0.7)
Purchases, issuances, sales, and settlements:    
Purchases 7.0 2.5
Sales 0.0 0.0
Settlements 0.0 (7.7)
Ending balance 57.9 36.6
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period 0.0 0.0
Equity securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 115.5 6.4
Transfers into Level 3 0.0 0.0
Transfers out of Level 3 0.0 0.0
Total gains or losses (realized/unrealized):    
Included in net income 2.0 28.3
Included in other comprehensive income 0.0 0.0
Purchases, issuances, sales, and settlements:    
Purchases 3.3 80.9
Sales 0.0 (0.1)
Settlements 0.0 0.0
Ending balance 120.8 115.5
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period (2.8) (28.4)
Other Investments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 4.5 0.0
Transfers into Level 3 4.3 0.0
Transfers out of Level 3 0.0 0.0
Total gains or losses (realized/unrealized):    
Included in net income 0.0 0.0
Included in other comprehensive income 0.0 0.0
Purchases, issuances, sales, and settlements:    
Purchases 1.0 8.3
Sales (0.5) (1.6)
Settlements 0.0 (2.2)
Ending balance 9.3 4.5
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period $ 0.0 $ 0.0