Investments - Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments
Net unrealized gains (losses) on fixed maturities, available-for-sale	$ 2,898.5	$ 1,727.8
Noncredit component of impairment losses on fixed maturities, available-for-sale	(93.3)	(146.4)
Net unrealized gains (losses) on equity securities, available-for-sale	2.0	(6.0)
Adjustments for assumed changes in amortization patterns	(150.6)	(121.9)
Adjustments for assumed changes in policyholder liabilities	(645.5)	(469.2)
Net unrealized gains (losses) on derivative instruments	108.2	186.5
Net unrealized gains (losses) on equity method subsidiaries and noncontrolling interest adjustments	31.3	68.0
Provision for deferred income tax benefits (taxes)	(695.5)	(411.8)
Net unrealized gains (losses) on available-for-sale securities and derivative instruments	$ 1,455.1	$ 827.0