Investments - Accumulated Credit Losses for Fixed Maturities with Bifurcated Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other-Than-Temporary Impairment Credit Losses Recognized in Net Income - Rollforward
Beginning balance	$ (139.9)	$ (131.5)	$ (144.4)
Credit losses for which an other-than-temporary impairment was not previously recognized	(15.0)	(43.4)	(6.1)
Credit losses for which an other-than-temporary impairment was previously recognized	(42.5)	(31.7)	(13.8)
Reduction for credit losses previously recognized on fixed maturities now sold, paid down or intended to be sold	63.3	60.5	24.7
Net reduction (increase) for positive changes in cash flows expected to be collected and amortization	10.0	6.4	7.5
Foreign currency translation adjustment	(0.2)	(0.2)	0.6
Ending balance	$ (124.3)	$ (139.9)	$ (131.5)