Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	$ 2,870.9	$ (2,314.5)	$ 721.7
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	2,736.5	5,677.3	4,664.6
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	3,627.9	(2,928.7)	1,012.7
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	2,736.5	5,677.3	4,664.6	$ 5,665.2
End of Year Balance	6,364.4	2,736.5	5,677.3
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	0.0	14.4	(22.7)
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	0.0	0.0	37.1
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	0.0	14.4	(22.7)	0.0
End of Year Balance	0.0	0.0	14.4
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(27.9)	(51.4)	(38.9)
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	(34.8)	23.5	(12.5)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(27.9)	(51.4)	(38.9)	(51.4)
End of Year Balance	(62.7)	(27.9)	(51.4)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(3,220.3)	(5,094.7)	(4,253.2)
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	(2,582.8)	1,874.4	(841.5)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(3,220.3)	(5,094.7)	(4,253.2)	(5,094.7)
End of Year Balance	(5,803.1)	(3,220.3)	(5,094.7)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	261.4	375.8	321.3
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	163.3	(114.4)	54.5
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	261.4	375.8	321.3	375.8
End of Year Balance	424.7	261.4	375.8
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(62.1)	(313.6)	(230.5)
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	(245.3)	242.5	(83.1)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(62.1)	(313.6)	(230.5)	(304.6)
End of Year Balance	(307.4)	(62.1)	(313.6)
Net Unrealized Gain (Loss) on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(312.4)	607.8	440.6
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $757.0; $(614.2); $328.1)	928.3	(902.7)	167.2
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(312.4)	607.8	440.6	$ 590.3
End of Year Balance	$ 615.9	$ (312.4)	$ 607.8