Investments - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Investment grade rating	92.50%
Reverse repurchase agreement, cash to value percentage	95.00%
Securities sold under agreements to repurchase	$ 45.0	$ 0.0
Fair Value	5,573.2	5,235.3
FHLB funding agreements	959.5	904.5
Fair Value
Schedule of Investments [Line Items]
Securities sold under agreements to repurchase	46.9	0.0
FHLB of Chicago
Schedule of Investments [Line Items]
Fair Value	1,030.2	987.2
Governmental Agencies as Required by Law in Various States
Schedule of Investments [Line Items]
Fair Value	$ 27.7	$ 29.2