Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill
Goodwill, Beginning Balance	$ 206,159	$ 153,374
Goodwill acquired during the year		52,785
Goodwill impairment	0	0	$ 0
Adjustments to identifiable assets acquired and liabilities assumed	8,944
Goodwill, Ending Balance	215,103	206,159	153,374
Power
Goodwill
Goodwill, Beginning Balance	25,933	24,391
Goodwill acquired during the year		1,542
Adjustments to identifiable assets acquired and liabilities assumed	261
Goodwill, Ending Balance	26,194	25,933	24,391
Pipeline
Goodwill
Goodwill, Beginning Balance	52,285	51,521
Goodwill acquired during the year		764
Adjustments to identifiable assets acquired and liabilities assumed	130
Goodwill, Ending Balance	52,415	52,285	51,521
Utilities
Goodwill
Goodwill, Beginning Balance	37,312	37,312
Goodwill, Ending Balance	37,312	37,312	37,312
Transmission
Goodwill
Goodwill, Beginning Balance	50,479
Goodwill acquired during the year		50,479
Adjustments to identifiable assets acquired and liabilities assumed	8,553
Goodwill, Ending Balance	59,032	50,479
Civil
Goodwill
Goodwill, Beginning Balance	40,150	40,150
Goodwill, Ending Balance	$ 40,150	$ 40,150	$ 40,150