Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain on Securities	$ 983.0	$ 2,870.9	$ (2,314.5)
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	6,364.4	2,736.5	5,677.3
Change in Net Unrealized Gain on Securities	1,233.2	3,627.9	(2,928.7)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	6,364.4	2,736.5	5,677.3	$ 5,665.2
End of Year Balance	7,597.6	6,364.4	2,736.5
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	0.0	0.0	14.4
Change in Net Unrealized Gain on Securities	0.0	0.0	0.0
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	0.0	0.0	14.4	0.0
End of Year Balance	0.0	0.0	0.0
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(62.7)	(27.9)	(51.4)
Change in Net Unrealized Gain on Securities	(22.4)	(34.8)	23.5
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(62.7)	(27.9)	(51.4)	(51.4)
End of Year Balance	(85.1)	(62.7)	(27.9)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(5,803.1)	(3,220.3)	(5,094.7)
Change in Net Unrealized Gain on Securities	(422.5)	(2,582.8)	1,874.4
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(5,803.1)	(3,220.3)	(5,094.7)	(5,094.7)
End of Year Balance	(6,225.6)	(5,803.1)	(3,220.3)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	424.7	261.4	375.8
Change in Net Unrealized Gain on Securities	(224.5)	163.3	(114.4)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	424.7	261.4	375.8	375.8
End of Year Balance	200.2	424.7	261.4
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	(307.4)	(62.1)	(313.6)
Change in Net Unrealized Gain on Securities	(112.0)	(245.3)	242.5
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	(307.4)	(62.1)	(313.6)	(304.6)
End of Year Balance	(419.4)	(307.4)	(62.1)
Net Unrealized Gain (Loss) on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Year Balance	615.9	(312.4)	607.8
Change in Net Unrealized Gain on Securities	451.8	928.3	(902.7)
Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax	615.9	(312.4)	607.8	$ 590.3
End of Year Balance	$ 1,067.7	$ 615.9	$ (312.4)