Segment Information - Summary of segment's internal financial reports (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Contract revenues	$ 583,214	$ 399,537	$ 1,500,084	$ 1,084,824
Income from operations	17,262	14,137	36,298	32,901
T&D [Member]
Segment Reporting Information [Line Items]
Contract revenues	294,940	222,531	823,398	635,842
Income from operations	17,726	13,935	48,706	38,494
C&I [Member]
Segment Reporting Information [Line Items]
Contract revenues	288,274	177,006	676,686	448,982
Income from operations	10,795	10,227	20,365	25,198
General Corporate [Member]
Segment Reporting Information [Line Items]
Income from operations	$ (11,259)	$ (10,025)	$ (32,773)	$ (30,791)