Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
Schedule of Investments [Line Items]
Investment grade rating	97.70%
Reverse repurchase agreement, cash to value percentage	95.00%
Securities sold under agreements to repurchase	$ 0.0	$ 0.0
Fair Value	5,780.1	5,714.6
FHLB funding agreements	1,054.5	1,039.5
FHLB of Chicago
Schedule of Investments [Line Items]
Fair Value	1,132.3	1,107.1
Governmental Agencies as Required by Law in Various States
Schedule of Investments [Line Items]
Fair Value	25.0	26.3
Fair Value
Schedule of Investments [Line Items]
Securities sold under agreements to repurchase	0.0	0.0
Fair Value	$ 5,780.1	$ 5,714.6