Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 67.0	$ 120.2	$ 173.0	$ 264.0
Other comprehensive (loss) income:
Change in unrealized gains on securities, net of taxes of $28.2, $(40.7), $76.0 and $(57.7), respectively	(105.9)	73.9	(281.1)	106.3
Change in unrealized gains on derivative transactions, net of taxes of $0.2 and $(5.4) for the three and six months ended June 30, 2018, respectively	(0.2)	0.0	20.9	0.0
Change in other-than-temporary impairment losses, net of taxes of $0.4, $0.8, $1.3 and $1.0, respectively	(1.4)	(1.5)	(4.9)	(1.8)
Change in foreign currency translation, net of taxes of $1.0, $(0.9), $1.5 and $(1.3), respectively	(83.1)	15.7	(73.9)	37.5
Amortization of pension and postretirement unrecognized net periodic benefit cost, net of taxes of $(0.3), $(0.3), $(0.3) and $(0.2), respectively	1.0	0.5	1.0	0.3
Total other comprehensive (loss) income	(189.6)	88.6	(338.0)	142.3
Total comprehensive (loss) income	$ (122.6)	$ 208.8	$ (165.0)	$ 406.3