Employee Retirement Plans - Amounts recognized in other comprehensive income (loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Actuarial gain (loss)	$ 5.2	$ (5.3)	$ (9.4)
Amortization of actuarial loss	4.9	5.1	5.0
Total before income taxes	10.1	(0.2)	(4.4)
Income tax expense (benefit)	3.1	(0.1)	(1.6)
Net amount recognized in other comprehensive income (loss)	$ 7.0	$ (0.1)	$ (2.8)