Derivative Financial Instruments - Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships	$ 31.1	$ (29.3)	$ 52.9	$ 14.2
Net gains (losses) expected to be reclassified from accumulated OCI into net income in the next 12 months			22.1
Interest rate contracts | Fixed maturities, available-for-sale
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships				(3.0)
Interest rate contracts | Investment contracts
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships	0.6		0.6
Foreign exchange contracts | Fixed maturities, available-for-sale
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships	$ 30.5	$ (29.3)	$ 52.3	$ 17.2