Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain (Loss) on Securities	$ 721.7	$ 661.9	$ (1,720.9)
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	4,664.6	3,695.7	6,261.5
Change in Net Unrealized Gain (Loss) on Securities	1,012.7	968.9	(2,565.8)
End of Year Balance	5,677.3	4,664.6	3,695.7
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(22.7)	(33.7)	13.9
Change in Net Unrealized Gain (Loss) on Securities	37.1	11.0	(47.6)
End of Year Balance	14.4	(22.7)	(33.7)
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(38.9)	(29.4)	(50.8)
Change in Net Unrealized Gain (Loss) on Securities	(12.5)	(9.5)	21.4
End of Year Balance	(51.4)	(38.9)	(29.4)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(4,253.2)	(3,578.4)	(6,150.3)
Change in Net Unrealized Gain (Loss) on Securities	(841.5)	(674.8)	2,571.9
End of Year Balance	(5,094.7)	(4,253.2)	(3,578.4)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	321.3	263.2	365.0
Change in Net Unrealized Gain (Loss) on Securities	54.5	58.1	(101.8)
End of Year Balance	375.8	321.3	263.2
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(230.5)	(113.1)	(149.0)
Change in Net Unrealized Gain (Loss) on Securities	(83.1)	(117.4)	35.9
End of Year Balance	(313.6)	(230.5)	(113.1)
Net Unrealized Gain on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	440.6	204.3	290.3
Change in Net Unrealized Gain (Loss) on Securities	167.2	236.3	(86.0)
End of Year Balance	$ 607.8	$ 440.6	$ 204.3