Investments - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Investment grade rating	97.20%
Reverse repurchase agreement, cash to value percentage	95.00%
Securities sold under agreements to repurchase	$ 0.2	$ 70.2
Fair Value	5,211.9	5,185.0
FHLB funding agreements	889.5	792.5
FHLB of Chicago
Schedule of Investments [Line Items]
Fair Value	961.6	860.4
Governmental Agencies as Required by Law in Various States
Schedule of Investments [Line Items]
Fair Value	28.8	28.6
Fair Value
Schedule of Investments [Line Items]
Securities sold under agreements to repurchase	$ 0.4	$ 73.3
Investment Grade
Schedule of Investments [Line Items]
Decrease in credit spread (percent)	0.12%
Increase in total returns (percent)	3.23%
Decrease to Bloomberg Barclays Index Yield-To-Worst (percent)	0.06%
Bloomberg Barclays Index Yield-to-Worst	5.48%
High Yield
Schedule of Investments [Line Items]
Decrease in credit spread (percent)	0.64%
Increase in total returns (percent)	5.42%
Decrease to Bloomberg Barclays Index Yield-To-Worst (percent)	0.46%
Bloomberg Barclays Index Yield-to-Worst	8.50%
10-year U.S. Treasury
Schedule of Investments [Line Items]
Decrease in interest rates	0.03%
Federal interest rate on securities (percent)	3.84%	3.87%