Derivative Financial Instruments - Cash Flow Hedges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships	$ (40.1)	$ (19.3)	$ 57.4
Net gains (losses) expected to be reclassified from accumulated OCI into net income in the next 12 months	23.5
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)	(9.9)	36.7
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	$ (37.1)	$ (9.4)	20.8
Foreign exchange contracts | Investment contracts
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships			$ (0.1)