Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in Components of Net Unrealized Gain on Securities
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	$ (2,314.5)	$ 721.7	$ 661.9
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	5,677.3	4,664.6	3,695.7
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Beginning Balance Adjustment, After-tax	(12.1)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	(2,928.7)	1,012.7	968.9
End of Year Balance	2,736.5	5,677.3	4,664.6
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	14.4	(22.7)	(33.7)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Beginning Balance Adjustment, After-tax	(14.4)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	0.0	37.1	11.0
End of Year Balance	0.0	14.4	(22.7)
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(51.4)	(38.9)	(29.4)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	23.5	(12.5)	(9.5)
End of Year Balance	(27.9)	(51.4)	(38.9)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(5,094.7)	(4,253.2)	(3,578.4)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	1,874.4	(841.5)	(674.8)
End of Year Balance	(3,220.3)	(5,094.7)	(4,253.2)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	375.8	321.3	263.2
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	(114.4)	54.5	58.1
End of Year Balance	261.4	375.8	321.3
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(313.6)	(230.5)	(113.1)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Beginning Balance Adjustment, After-tax	9.0
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	242.5	(83.1)	(117.4)
End of Year Balance	(62.1)	(313.6)	(230.5)
Net Unrealized Gain (Loss) on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	607.8	440.6	204.3
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Beginning Balance Adjustment, After-tax	(17.5)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	(902.7)	167.2	236.3
End of Year Balance	$ (312.4)	$ 607.8	$ 440.6