v3.26.3
Investments - Narrative (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2026
Dec. 31, 2025
Schedule of Investments [Line Items]    
Investment grade rating 98.20%  
Reverse repurchase agreement, cash to value percentage 95.00%  
Securities sold under agreements to repurchase $ 0.0 $ 0.0
Fair Value 5,520.8 5,714.6
FHLB funding agreements 1,084.5 1,039.5
FHLB of Chicago    
Schedule of Investments [Line Items]    
Fair Value 1,166.2 1,107.1
Governmental Agencies as Required by Law in Various States    
Schedule of Investments [Line Items]    
Fair Value 24.4 26.3
Fair Value    
Schedule of Investments [Line Items]    
Securities sold under agreements to repurchase 0.0 0.0
Fair Value $ 5,520.8 $ 5,714.6