Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain on Securities	$ (1,042.1)	$ 176.9
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	5,665.2	4,664.6
Change in Net Unrealized Gain on Securities	(1,317.0)	261.0
End of Period Balance	4,348.2	4,925.6
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance		(22.7)
Change in Net Unrealized Gain on Securities		4.9
End of Period Balance		(17.8)
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(51.4)	(38.9)
Change in Net Unrealized Gain on Securities	10.2	(2.0)
End of Period Balance	(41.2)	(40.9)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(5,094.7)	(4,253.2)
Change in Net Unrealized Gain on Securities	1,064.6	(214.1)
End of Period Balance	(4,030.1)	(4,467.3)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	375.8	321.3
Change in Net Unrealized Gain on Securities	(54.8)	14.3
End of Period Balance	321.0	335.6
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(304.6)	(230.5)
Change in Net Unrealized Gain on Securities	62.8	(21.7)
End of Period Balance	(241.8)	(252.2)
Net Unrealized Gain on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	590.3	440.6
Change in Net Unrealized Gain on Securities	(234.2)	42.4
End of Period Balance	$ 356.1	$ 483.0