v3.26.1
Investments - Amortized Cost, Allowance for Credit Losses, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost $ 9,133.1 $ 8,635.3
Fixed maturity securities available for sale, allowances for credit losses 0.0 (1.9)
Fixed maturity securities, gross unrealized gains 98.4 174.9
Fixed maturity securities, gross unrealized losses (242.0) (230.6)
Fixed maturity securities, fair value 8,989.5 8,577.7
U.S. government and government agencies and authorities    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 63.9 62.7
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 0.2 1.0
Fixed maturity securities, gross unrealized losses (1.3) (0.8)
Fixed maturity securities, fair value 62.8 62.9
States, municipalities and political subdivisions    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 98.3 100.0
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 0.7 1.2
Fixed maturity securities, gross unrealized losses (5.3) (5.1)
Fixed maturity securities, fair value 93.7 96.1
Foreign governments    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 647.5 596.1
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 8.1 9.1
Fixed maturity securities, gross unrealized losses (13.0) (11.3)
Fixed maturity securities, fair value 642.6 593.9
Asset-backed    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 861.1 850.2
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 4.3 4.6
Fixed maturity securities, gross unrealized losses (8.6) (9.1)
Fixed maturity securities, fair value 856.8 845.7
Commercial mortgage-backed    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 482.9 431.4
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 2.7 5.3
Fixed maturity securities, gross unrealized losses (18.6) (19.3)
Fixed maturity securities, fair value 467.0 417.4
Residential mortgage-backed    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 1,113.9 978.6
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 6.0 11.5
Fixed maturity securities, gross unrealized losses (38.7) (35.4)
Fixed maturity securities, fair value 1,081.2 954.7
U.S. corporate    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 4,007.8 3,895.5
Fixed maturity securities available for sale, allowances for credit losses 0.0 (1.9)
Fixed maturity securities, gross unrealized gains 48.0 97.3
Fixed maturity securities, gross unrealized losses (119.6) (113.3)
Fixed maturity securities, fair value 3,936.2 3,877.6
Foreign corporate    
Debt Securities, Available-for-sale [Line Items]    
Fixed maturity securities, cost or amortized cost 1,857.7 1,720.8
Fixed maturity securities available for sale, allowances for credit losses 0.0 0.0
Fixed maturity securities, gross unrealized gains 28.4 44.9
Fixed maturity securities, gross unrealized losses (36.9) (36.3)
Fixed maturity securities, fair value $ 1,849.2 $ 1,729.4