Accumulated Other Comprehensive Loss Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Change in Components of Net Unrealized Gain (Loss) on Securities
Change in Net Unrealized Gain on Securities	$ 1,047.7	$ (635.0)	$ 2,292.8	$ (1,677.1)
Net Unrealized Gain (Loss) on Securities
Change in Components of Net Unrealized Gain (Loss) on Securities
Beginning of Period Balance	(0.1)	356.1	(312.4)	590.3
Change in Net Unrealized Gain on Securities	287.0	(131.6)	599.3	(365.8)
End of Period Balance	286.9	224.5	286.9	224.5
Fixed Maturity Securities
Change in Components of Net Unrealized Gain (Loss) on Securities
Beginning of Period Balance	4,308.4	4,348.2	2,736.5	5,665.2
Change in Net Unrealized Gain on Securities	1,327.0	(804.5)	2,898.9	(2,121.5)
End of Period Balance	5,635.4	3,543.7	5,635.4	3,543.7
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain (Loss) on Securities
Beginning of Period Balance	(42.6)	(41.2)	(27.9)	(51.4)
Change in Net Unrealized Gain on Securities	(14.4)	7.1	(29.1)	17.3
End of Period Balance	(57.0)	(34.1)	(57.0)	(34.1)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain (Loss) on Securities
Beginning of Period Balance	(4,451.0)	(4,030.1)	(3,220.3)	(5,094.7)
Change in Net Unrealized Gain on Securities	(1,004.7)	663.0	(2,235.4)	1,727.6
End of Period Balance	(5,455.7)	(3,367.1)	(5,455.7)	(3,367.1)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain (Loss) on Securities
Beginning of Period Balance	328.5	321.0	261.4	375.8
Change in Net Unrealized Gain on Securities	57.2	(32.9)	124.3	(87.7)
End of Period Balance	385.7	288.1	385.7	288.1
Income Tax
Change in Components of Net Unrealized Gain (Loss) on Securities
Beginning of Period Balance	(143.4)	(241.8)	(62.1)	(304.6)
Change in Net Unrealized Gain on Securities	(78.1)	35.7	(159.4)	98.5
End of Period Balance	$ (221.5)	$ (206.1)	$ (221.5)	$ (206.1)