Consolidated Statements of Comprehensive Income (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]
Adjustment of and amortization of net actuarial loss and prior service cost, taxes	$ 0.1	$ 0.0	$ 0.3	$ 0.0
Other comprehensive income, equity method investment, tax		$ 0.0		$ 0.0