Short-Term and Long-Term Debt - Carrying Values (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Long Term and Short Term Debt Instruments [Line Items]
Short-term debt	$ 1,601.7	$ 0.0
Total long-term debt	1,068.5	322.0
Term loan due 2022 | Term loan
Schedule of Long Term and Short Term Debt Instruments [Line Items]
Short-term debt	1,601.7	0.0
Term loan due 2023 | Term loan
Schedule of Long Term and Short Term Debt Instruments [Line Items]
Total long-term debt	750.8	0.0
Surplus notes
Schedule of Long Term and Short Term Debt Instruments [Line Items]
Total long-term debt	249.7	249.7
FHLBI bank loans
Schedule of Long Term and Short Term Debt Instruments [Line Items]
Total long-term debt	$ 68.0	$ 72.3