Reportable Segments (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)
Segment reporting information
Number of reportable segments | segment										3
Revenue	$ 579,017	$ 608,311	$ 631,165	$ 561,502	$ 601,863	$ 507,828	$ 456,811	$ 430,446	$ 2,379,995	$ 1,996,948	$ 1,929,415
% of Total Revenue									100.00%	100.00%	100.00%
Gross Profit	$ 68,477	$ 70,421	$ 84,483	$ 55,053	$ 68,616	$ 50,129	$ 43,285	$ 39,277	$ 278,434	$ 201,307	$ 219,873
% of Revenue									11.70%	10.10%	11.40%
Power
Segment reporting information
Revenue									$ 606,125	$ 478,653	$ 466,292
% of Total Revenue									25.50%	24.00%	24.20%
Gross Profit									$ 65,675	$ 49,807	$ 53,620
% of Revenue									10.80%	10.40%	11.50%
Pipeline
Segment reporting information
Revenue									$ 465,570	$ 401,931	$ 299,365
% of Total Revenue									19.50%	20.10%	15.50%
Gross Profit									$ 92,087	$ 68,100	$ 24,685
% of Revenue									19.80%	16.90%	8.20%
Utilities
Segment reporting information
Revenue									$ 806,523	$ 637,212	$ 587,047
% of Total Revenue									33.90%	31.90%	30.40%
Gross Profit									$ 113,037	$ 100,071	$ 96,450
% of Revenue									14.00%	15.70%	16.40%
Civil
Segment reporting information
Revenue									$ 501,777	$ 479,152	$ 576,711
% of Total Revenue									21.10%	24.00%	29.90%
Gross Profit									$ 7,635	$ (16,671)	$ 45,118
% of Revenue									1.50%	(3.50%)	7.80%