Derivative Financial Instruments - Fair Value Hedges (Details) - Fixed maturities, available-for-sale - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value Hedges
Amortized cost of hedged item - Active hedging relationships	$ 829.2	$ 476.1
Amortized cost of hedged item - Discontinued hedging relationships	80.2	135.1
Amortized cost of hedged item - Active or discontinued hedging relationships	909.4	611.2
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	1.0	21.4
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	3.1	5.2
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	4.1	26.6
Amortized cost basis of closed portfolio used in last-of-layer hedging relationship	347.7	0.0
Cumulative basis adjustments associated with last-of-layer hedging relationship	(1.9)	0.0
Amount of hedged item in last-of-layer hedging relationship	$ 210.0	$ 0.0