Consolidated Statements of Comprehensive Income (unaudited) (Parenthetical) - 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 change in unrealized gains on securities, taxes	$ 28.2	$ (40.7)	$ 76.0	$ (57.7)
Net change in unrealized gains on derivative transactions, taxes	0.2	0.0	(5.4)	0.0
Net change in other-than-temporary impairment gains, taxes	0.4	0.8	1.3	1.0
Net change in foreign currency translation, taxes	1.0	(0.9)	1.5	(1.3)
Net change in pension plan curtailment and amortization of pension and postretirement unrecognized net periodic benefit cost, taxes	$ (0.3)	$ (0.3)	$ (0.3)	$ (0.2)