Employee Retirement Plans (Multiemployer Pension Plans) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) plan	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Multiemployer Plans [Line Items]
Number of underfunded plans | plan	2
Multiemployer plan contributions	$ 10.5	$ 20.4
Withdrawal obligation	20.4
Other Long-Term Liabilities
Multiemployer Plans [Line Items]
Withdrawal obligation	13.5		$ 19.4
Accrued Liabilities
Multiemployer Plans [Line Items]
Withdrawal obligation	$ 6.9		$ 8.8