Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 143.8	$ 220.4
Other comprehensive income (loss):
Change in unrealized gains on securities, net of taxes of $(17.0) and $(14.8), respectively	32.4	29.9
Change in other-than-temporary impairment gains, net of taxes of $0.2 and $0.7, respectively	(0.3)	(1.3)
Change in foreign currency translation, net of taxes of $(0.4) and $(1.6), respectively	21.8	11.8
Amortization of pension and postretirement unrecognized net periodic benefit cost, net of taxes of $0.1 and $(35.7), respectively	(0.2)	66.2
Total other comprehensive income	53.7	106.6
Total comprehensive income	$ 197.5	$ 327.0