CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 994.2	$ 931.4	$ 867.1
Other Comprehensive Income (Loss)
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $328.1; $318.0; $(892.5))	721.7	661.9	(1,720.9)
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance (net of tax expense (benefit) of $(245.0); $(200.6); $856.6)	(554.5)	(425.6)	1,634.9
Change in Net Gain on Cash Flow Hedges (net of tax benefit of $24.4; $25.4; $4.3)	(45.2)	(50.5)	(13.0)
Change in Foreign Currency Translation Adjustment (net of tax benefit of $-; $-; $0.1)	99.5	(180.4)	(60.2)
Change in Unrecognized Pension and Postretirement Benefit Costs (net of tax expense (benefit) of $(8.3); $(34.2); $3.2)	(43.0)	(72.5)	8.9
Total Other Comprehensive Income (Loss)	178.5	(67.1)	(150.3)
Comprehensive Income	$ 1,172.7	$ 864.3	$ 716.8