Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Securities sold under agreements to repurchase	$ 50.0	$ 0.0
Reverse repurchase agreement, cash to value percentage	95.00%
Investment grade rating	95.90%
Fair Value	$ 5,342.7	5,235.3
FHLB funding agreements	929.5	904.5
Fair Value
Schedule of Investments [Line Items]
Securities sold under agreements to repurchase	51.8	0.0
FHLB of Chicago
Schedule of Investments [Line Items]
Fair Value	1,014.4	987.2
Governmental Agencies as Required by Law in Various States
Schedule of Investments [Line Items]
Fair Value	$ 27.0	$ 29.2