Supplemental Balance Sheet Information - Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Supplemental Balance Sheet Data [Abstract]
Vendor and other accrued expenses	$ 35,193	$ 33,108
Estimated casualty and healthcare liabilities	14,772	14,610
Job costs	9,585	8,707
Accrued compensation	27,901	23,398
Income taxes payable	4,560	1,870
Deferred income taxes	0	3,317
Total	$ 92,011	$ 85,010