Consolidated Statements Of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Change in unrealized gains on securities, net of taxes of $(66.3), $18.9, and $158.6, respectively	$ (66.3)	$ 18.9	$ 158.6
Change in other-than-temporary impairment gains, net of taxes of $1.5, $1.0, and $2.2, respectively	1.5	1.0	2.2
Change in foreign currency translation, net of taxes of $(2.3), $(0.4), and $5.1, respectively	(2.3)	(0.4)	5.1
Amortization of pension and postretirement unrecognized net periodic benefit cost and change in funded status, net of taxes of $11.0, $(35.2), and $(4.1), respectively	$ 11.0	$ (35.2)	$ (4.1)