Investments - Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments
Net unrealized gains (losses) on fixed maturities, available-for-sale	$ 5,749.7	$ 8,193.0
Net unrealized gains (losses) on derivative instruments	79.6	38.9
Adjustments for assumed changes in amortization patterns	(316.5)	(437.3)
Adjustments for assumed changes in policyholder liabilities	(1,029.2)	(2,603.9)
Net unrealized gains (losses) on other investments and noncontrolling interest adjustments	55.9	78.0
Provision for deferred income tax benefits (taxes)	(956.7)	(1,112.2)
Net unrealized gains (losses) on available-for-sale securities and derivative instruments	$ 3,582.8	$ 4,156.5