Reportable Segments (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment reporting information
Number of reportable segments | segment			5
Revenue	$ 908,902	$ 608,311	$ 2,061,808	$ 1,800,978
% of Total Revenue	100.00%	100.00%	100.00%	100.00%
Gross Profit	$ 106,505	$ 70,421	$ 222,484	$ 209,957
% of Revenue	11.70%	11.60%	10.80%	11.70%
Power
Segment reporting information
Revenue	$ 181,822	$ 154,178	$ 515,378	$ 443,191
% of Total Revenue	20.00%	25.30%	25.00%	24.60%
Gross Profit	$ 32,077	$ 18,842	$ 76,674	$ 52,498
% of Revenue	17.60%	12.20%	14.90%	11.80%
Pipeline
Segment reporting information
Revenue	$ 213,073	$ 84,357	$ 361,261	$ 402,425
% of Total Revenue	23.40%	13.90%	17.50%	22.40%
Gross Profit	$ 24,999	$ 12,084	$ 43,568	$ 79,575
% of Revenue	11.70%	14.30%	12.10%	19.80%
Utilities
Segment reporting information
Revenue	$ 269,652	$ 246,524	$ 665,214	$ 576,446
% of Total Revenue	29.70%	40.50%	32.30%	32.00%
Gross Profit	$ 35,348	$ 36,081	$ 78,963	$ 76,701
% of Revenue	13.10%	14.60%	11.90%	13.30%
Transmission
Segment reporting information
Revenue	$ 121,526		$ 163,980
% of Total Revenue	13.40%	0.00%	7.90%	0.00%
Gross Profit	$ 13,958		$ 19,679
% of Revenue	11.50%	0.00%	12.00%	0.00%
Civil
Segment reporting information
Revenue	$ 122,829	$ 123,252	$ 355,975	$ 378,916
% of Total Revenue	13.50%	20.30%	17.30%	21.00%
Gross Profit	$ 123	$ 3,414	$ 3,600	$ 1,183
% of Revenue	0.10%	2.80%	1.00%	0.30%