CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Postretirement medical plan changes, income tax expense	$ 0.0	$ 0.0	$ (1.0)	$ 0.0
Pension and other postretirement plans adjustments:
Amortization of prior service cost, income tax expense	(0.1)	(0.1)	(0.3)	(0.4)
Amortization of net actuarial loss, income tax expense	$ 0.0	$ (0.1)	$ (0.1)	$ (0.2)