Reportable Segments (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment reporting information
Number of reportable segments | segment									5
Revenue	$ 789,778	$ 865,064	$ 789,929	$ 661,558	$ 877,670	$ 908,902	$ 648,787	$ 504,119	$ 3,106,329	$ 2,939,478	$ 2,379,995
% of Total Revenue									100.00%	100.00%	100.00%
Gross Profit	$ 89,514	$ 108,421	$ 80,531	$ 52,460	$ 103,253	$ 106,505	$ 71,419	$ 44,560	$ 330,926	$ 325,737	$ 278,434
% of Revenue									10.70%	11.10%	11.70%
Power
Segment reporting information
Revenue									$ 729,348	$ 694,048	$ 606,125
% of Total Revenue									23.50%	23.60%	25.50%
Gross Profit									$ 76,119	$ 109,789	$ 65,675
% of Revenue									10.40%	15.80%	10.80%
Pipeline
Segment reporting information
Revenue									$ 505,156	$ 590,937	$ 465,570
% of Total Revenue									16.30%	20.10%	19.50%
Gross Profit									$ 61,550	$ 66,602	$ 92,087
% of Revenue									12.20%	11.30%	19.80%
Utilities
Segment reporting information
Revenue									$ 886,504	$ 902,772	$ 806,523
% of Total Revenue									28.50%	30.70%	33.90%
Gross Profit									$ 116,645	$ 111,825	$ 113,037
% of Revenue									13.20%	12.40%	14.00%
Transmission
Segment reporting information
Revenue									$ 497,302	$ 286,749
% of Total Revenue									16.00%	9.80%
Gross Profit									$ 22,580	$ 31,904
% of Revenue									4.50%	11.10%
Civil
Segment reporting information
Revenue									$ 488,019	$ 464,972	$ 501,777
% of Total Revenue									15.70%	15.80%	21.10%
Gross Profit									$ 54,032	$ 5,617	$ 7,635
% of Revenue									11.10%	1.20%	1.50%