CONDENSED 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]
Defined benefit pension and other postretirement plan adjustments, tax	$ 0.4	$ (0.3)	$ 0.1	$ (0.4)
Net loss on investment securities, tax	0.0	(3.0)	0.0	(2.9)
Net gain (loss) on derivatives, tax	$ 0.3	$ (1.1)	$ 1.1	$ (0.3)