Investments - Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments
Net unrealized gains (losses) on fixed maturities, available-for-sale	$ 8,193.0	$ 4,834.2
Noncredit component of impairment losses on fixed maturities, available-for-sale		(48.4)
Net unrealized gains (losses) on derivative instruments	38.9	94.1
Adjustments for assumed changes in amortization patterns	(437.3)	(261.0)
Adjustments for assumed changes in policyholder liabilities	(2,603.9)	(1,133.5)
Net unrealized gains (losses) on other investments and noncontrolling interest adjustments	78.0	96.8
Provision for deferred income tax benefits (taxes)	(1,112.2)	(766.9)
Net unrealized gains (losses) on available-for-sale securities and derivative instruments	$ 4,156.5	$ 2,815.3