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Segment Information - Summary of segment's internal financial reports (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	[1]	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Contract revenues	$ 446,345		$ 399,537	$ 339,676	$ 345,611	$ 373,501		$ 373,502	$ 356,185	$ 300,129	$ 1,531,169	$ 1,403,317	$ 1,142,487
Income from operations											50,312	29,558	38,754
T&D [Member]
Segment Reporting Information [Line Items]
Contract revenues											893,108	879,372	818,972
Income from operations											57,242	39,631	63,459
C&I [Member]
Segment Reporting Information [Line Items]
Contract revenues											638,061	523,945	323,515
Income from operations											34,112	25,048	13,920
General Corporate [Member]
Segment Reporting Information [Line Items]
Income from operations											$ (41,042)	$ (35,121)	$ (38,625)
[1] Results for the fourth quarter of 2017 include a net 2017 Tax Act benefit of $7.8 million, or $0.48 and $0.47 per basic and diluted share, respectively.

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