Nature of Operations and Significant Accounting Policies - Pre-tax Impacts To Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Increase to income before taxes
Cost of reinsurance amortization	$ 25.0	$ (7.6)	$ (8.2)	$ (2.6)
Income (loss) before income taxes	$ 1,604.9	$ 1,640.4	$ 1,846.0	5,989.4
Adjustment related to actuarial balance re-cohorting
Increase to income before taxes
Cost of reinsurance amortization				33.7
Change in additional liability for certain benefit features				167.4
Income (loss) before income taxes				201.1
Increase to pre-tax other comprehensive income
Cost of reinsurance unrealized gains (losses)				(2.1)
Change in additional liability for certain benefit features unrealized gains				7.8
Total increase to pre-tax other comprehensive income				5.7
Total increase to pre-tax comprehensive income				$ 206.8