Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain (Loss) on Securities	$ (305.0)	$ 66.1	$ (1,982.1)	$ 616.3
Net Unrealized Gain (Loss) on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	224.5	558.2	590.3	440.6
Change in Net Unrealized Gain (Loss) on Securities	(565.2)	(0.8)	(931.0)	116.8
End of Period Balance	(340.7)	557.4	(340.7)	557.4
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	3,543.7	5,492.2	5,665.2	4,664.6
Change in Net Unrealized Gain (Loss) on Securities	(385.6)	95.5	(2,507.1)	923.1
End of Period Balance	3,158.1	5,587.7	3,158.1	5,587.7
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance		(4.6)		(22.7)
Change in Net Unrealized Gain (Loss) on Securities		9.8		27.9
End of Period Balance		5.2		5.2
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(34.1)	(45.8)	(51.4)	(38.9)
Change in Net Unrealized Gain (Loss) on Securities	2.6	(0.4)	19.9	(7.3)
End of Period Balance	(31.5)	(46.2)	(31.5)	(46.2)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(3,367.1)	(4,948.2)	(5,094.7)	(4,253.2)
Change in Net Unrealized Gain (Loss) on Securities	(317.8)	(109.2)	1,409.8	(804.2)
End of Period Balance	(3,684.9)	(5,057.4)	(3,684.9)	(5,057.4)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	288.1	360.7	375.8	321.3
Change in Net Unrealized Gain (Loss) on Securities	(14.8)	10.9	(102.5)	50.3
End of Period Balance	273.3	371.6	273.3	371.6
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(206.1)	(296.1)	(304.6)	(230.5)
Change in Net Unrealized Gain (Loss) on Securities	150.4	(7.4)	248.9	(73.0)
End of Period Balance	$ (55.7)	$ (303.5)	$ (55.7)	$ (303.5)