Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Net Investment Income [Line Items]
Investment grade rating	97.90%
Reverse repurchase agreement, cash to value percentage	95.00%
Securities sold under agreements to repurchase	$ 12.0	$ 0.0
Fair Value	5,387.9	5,235.3
FHLB funding agreements	989.5	904.5
Fair Value
Net Investment Income [Line Items]
Securities sold under agreements to repurchase	12.4	0.0
FHLB of Chicago
Net Investment Income [Line Items]
Fair Value	1,072.2	987.2
Governmental agencies as required by law in various states
Net Investment Income [Line Items]
Fair Value	$ 27.1	$ 29.2