Derivative Financial Instruments - Fair Value Hedges (Details) - Fixed maturities, available-for-sale - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Fair Value Hedges
Amortized cost of hedged item - Active hedging relationships	$ 3,454.0	$ 3,498.6
Amortized cost of hedged item - Discontinued hedging relationships	75.5	48.8
Amortized cost of hedged item - Active or discontinued hedging relationships	3,529.5	3,547.4
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	(126.1)	(153.4)
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	0.3	1.3
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	(125.8)	(152.1)
Amortized cost basis of closed portfolio used in last-of-layer hedging relationship	3,215.0	3,256.9
Cumulative basis adjustments associated with last-of-layer hedging relationship	(83.5)	(102.4)
Amount of hedged item in last-of-layer hedging relationship	$ 1,110.0	$ 1,110.0