Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net change in unrealized gains on securities, taxes	$ (17.0)	$ (14.8)
Net change in other-than-temporary impairment gains, taxes	0.2	0.7
Net change in foreign currency translation, taxes	(0.4)	(1.6)
Net change in pension plan curtailment and amortization of pension and postretirement unrecognized net periodic benefit cost, taxes	$ 0.1	$ (35.7)