Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain on Securities	$ (635.0)	$ 373.3	$ (1,677.1)	$ 550.2
Net Unrealized Gain on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	356.1	483.0	590.3	440.6
Change in Net Unrealized Gain on Securities	(131.6)	75.2	(365.8)	117.6
End of Period Balance	224.5	558.2	224.5	558.2
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	4,348.2	4,925.6	5,665.2	4,664.6
Change in Net Unrealized Gain on Securities	(804.5)	566.6	(2,121.5)	827.6
End of Period Balance	3,543.7	5,492.2	3,543.7	5,492.2
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance		(17.8)		(22.7)
Change in Net Unrealized Gain on Securities		13.2		18.1
End of Period Balance		(4.6)		(4.6)
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(41.2)	(40.9)	(51.4)	(38.9)
Change in Net Unrealized Gain on Securities	7.1	(4.9)	17.3	(6.9)
End of Period Balance	(34.1)	(45.8)	(34.1)	(45.8)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(4,030.1)	(4,467.3)	(5,094.7)	(4,253.2)
Change in Net Unrealized Gain on Securities	663.0	(480.9)	1,727.6	(695.0)
End of Period Balance	(3,367.1)	(4,948.2)	(3,367.1)	(4,948.2)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	321.0	335.6	375.8	321.3
Change in Net Unrealized Gain on Securities	(32.9)	25.1	(87.7)	39.4
End of Period Balance	288.1	360.7	288.1	360.7
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(241.8)	(252.2)	(304.6)	(230.5)
Change in Net Unrealized Gain on Securities	35.7	(43.9)	98.5	(65.6)
End of Period Balance	$ (206.1)	$ (296.1)	$ (206.1)	$ (296.1)