Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Investment grade rating	96.30%
Reverse repurchase agreement, cash to value percentage	95.00%
Securities sold under agreements to repurchase	$ 70.2	$ 70.2
Fair Value	5,350.0	5,185.0
FHLB funding agreements	869.5	792.5
Fair Value
Schedule of Investments [Line Items]
Securities sold under agreements to repurchase	74.7	73.3
FHLB of Chicago
Schedule of Investments [Line Items]
Fair Value	948.1	860.4
Governmental Agencies as Required by Law in Various States
Schedule of Investments [Line Items]
Fair Value	$ 29.0	$ 28.6
Investment Grade
Schedule of Investments [Line Items]
Increase in credit spread (percent)	0.46%
Decrease in total returns (percent)	12.80%
Increase to Bloomberg Barclays Index Yield-To-Worst (percent)	2.84%
Bloomberg Barclays Index Yield-to-Worst	5.20%
High Yield
Schedule of Investments [Line Items]
Increase in credit spread (percent)	1.72%
Decrease in total returns (percent)	8.00%
Increase to Bloomberg Barclays Index Yield-To-Worst (percent)	4.31%
Bloomberg Barclays Index Yield-to-Worst	8.50%
10-year U.S. Treasury
Schedule of Investments [Line Items]
Increase in interest rates	1.96%
Federal interest rate on securities (percent)	3.47%		1.51%