Multiemployer Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Multiemployer Benefit Plans
Multiemployer plans contributions represent more than 5% of total employer contributions	false	false	false
Multiemployer Pension Plans
Multiemployer Benefit Plans
Multiemployer plan, period contributions	$ 10,113	$ 9,479	$ 10,878
Multiemployer Health Plans
Multiemployer Benefit Plans
Multiemployer plan, period contributions	$ 29,549	$ 32,998	$ 29,269