Consolidated Statement Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 519.6	$ 565.4	$ 141.6
Other comprehensive income (loss):
Change in unrealized gains on securities, net of taxes of $(66.3), $18.9, and $158.6, respectively	121.9	(36.2)	(297.6)
Change in other-than-temporary impairment gains, net of taxes of $1.5, $1.0, and $2.2, respectively	(2.7)	(1.8)	(4.2)
Change in foreign currency translation, net of taxes of $(2.3), $(0.4), and $5.1, respectively	40.6	(51.4)	(143.0)
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	(20.4)	65.4	7.6
Total other comprehensive income (loss)	139.4	(24.0)	(437.2)
Total comprehensive income (loss)	$ 659.0	$ 541.4	$ (295.6)