Derivative Financial Instruments - Fair Value Hedges (Details) - Fixed maturities, available-for-sale - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value Hedges
Amortized cost of hedged item - Active hedging relationships	$ 482.3	$ 142.0
Amortized cost of hedged item - Discontinued hedging relationships	135.8	159.3
Amortized cost of hedged item - Active or discontinued hedging relationships	618.1	301.3
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	27.2	18.1
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	5.8	7.7
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	$ 33.0	$ 25.8