CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Change in Net Unrealized Gain on Securities Before Adjustment, Tax Expense (Benefit)	$ 328.1	$ 318.0	$ (892.5)
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance, Tax Expense (Benefit)	(245.0)	(200.6)	856.6
Change in Net Gain on Cash Flow Hedges, Tax Benefit	24.4	25.4	4.3
Change in Foreign Currency Translation Adjustment, Tax Benefit	0.0	0.0	0.1
Change in Unrecognized Pension and Postretirement Benefit Costs, Tax Expense (Benefit)	$ (8.3)	$ (34.2)	$ 3.2