Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Goodwill, Beginning Balance			$ 127,226	$ 124,161
Goodwill acquired during the year			26,269	5,781
Goodwill impairment	$ (2,716)		0	(2,716)	$ (401)
Purchase price allocation adjustments			(121)
Goodwill, Ending Balance		$ 124,161	153,374	127,226	124,161
Power
Goodwill
Goodwill, Beginning Balance			24,512	20,731
Goodwill acquired during the year				3,781
Purchase price allocation adjustments			(121)
Goodwill, Ending Balance		20,731	24,391	24,512	20,731
Pipeline
Goodwill
Goodwill, Beginning Balance			42,252	42,252
Goodwill acquired during the year			9,269
Goodwill, Ending Balance		42,252	51,521	42,252	42,252
Utilities
Goodwill
Goodwill, Beginning Balance			20,312	18,312
Goodwill acquired during the year			17,000	2,000
Goodwill, Ending Balance		18,312	37,312	20,312	18,312
Civil
Goodwill
Goodwill, Beginning Balance			40,150	42,866
Goodwill impairment				(2,716)
Goodwill, Ending Balance		42,866	$ 40,150	$ 40,150	$ 42,866
Cardinal Contractors
Goodwill
Goodwill impairment		$ (400)