Derivative Financial Instruments - Fair Value Hedges (Details) - Fixed maturities, available-for-sale - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value Hedges
Amortized cost of hedged item - Active hedging relationships	$ 142.0
Amortized cost of hedged item - Active hedging relationships		$ 137.0
Amortized cost of hedged item - Discontinued hedging relationships	159.3	298.9
Amortized cost of hedged item - Active or discontinued hedging relationships	301.3	435.9
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	18.1	12.4
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	7.7	13.3
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	$ 25.8	$ 25.7