Investments - Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments
Net unrealized gains on fixed maturities, available-for-sale	$ 2,026.1	$ 4,834.2
Noncredit component of impairment losses on fixed maturities, available-for-sale		(48.4)
Net unrealized gains on derivative instruments	154.7	94.1
Adjustments for assumed changes in amortization patterns	71.2	(261.0)
Adjustments for assumed changes in policyholder liabilities	(470.1)	(1,133.5)
Net unrealized gains on other investments and noncontrolling interest adjustments	69.6	96.8
Provision for deferred income taxes	(398.9)	(766.9)
Net unrealized gains on available-for-sale securities and derivative instruments	$ 1,452.6	$ 2,815.3