Investments - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Investment grade rating	96.00%
Fair Value	$ 5,235.3	$ 5,185.0	[1]
FHLB funding agreements	904.5	792.5
FHLB of Chicago
Net Investment Income [Line Items]
Fair Value	987.2	860.4
Governmental agencies as required by law in various states
Net Investment Income [Line Items]
Fair Value	$ 29.2	$ 28.6

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements