Derivative Financial Instruments - Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Cash Flow Hedges
Amount of gain (loss) recognized in AOCI on derivatives in cash flow hedging relationships	$ (26.2)	$ 4.3	$ 43.5	$ (8.5)
Net gains (losses) expected to be reclassified from accumulated OCI into net income in the next 12 months			24.8
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	(7.4)	0.9	(3.0)	(3.3)
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	$ (18.8)	$ 3.4	$ 46.5	$ (5.2)