Accounts Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Receivables [Abstract]
Trade accounts receivable	$ 78,858	$ 81,193
Contract receivables:
Amounts billed	61,351	41,569
Costs in excess of billings	11,610	6,582
Total contract receivables	72,961	48,151
Total accounts receivable	151,819	129,344
Less allowance for doubtful accounts	(6,434)	(5,272)	$ (4,868)	$ (4,280)
Accounts receivable	$ 145,385	$ 124,072