Employee Benefit Plans - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension And Other Postretirement Benefit Contributions
Accrued pension benefits	$ 46.9	$ 42.0	$ 38.0
Net actuarial gain (loss) recognized in accumulated other comprehensive loss (before tax)	1.5	(5.1)
Accumulated benefit obligation	47.1	41.3
Projected employer contributions for following year	$ 0.6	$ 0.5