Revenue Recognition - Schedule of revenue by contract type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Amount	$ 583,214	$ 399,537	$ 1,500,084	$ 1,084,824
Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 583,214	$ 399,537	$ 1,500,084	$ 1,084,824
Percent	100.00%	100.00%	100.00%	100.00%
Fixed-price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 378,229	$ 214,497	$ 879,953	$ 550,756
Percent	64.90%	53.70%	58.70%	50.80%
Unit Price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 68,910	$ 57,007	$ 195,109	$ 169,372
Percent	11.80%	14.30%	13.00%	15.60%
TE Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 117,167	$ 85,951	$ 320,040	$ 252,768
Percent	20.10%	21.50%	21.30%	23.30%
Other Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 18,908	$ 42,082	$ 104,982	$ 111,928
Percent	3.20%	10.50%	7.00%	10.30%
T&D [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 294,940	$ 222,531	$ 823,398	$ 635,842
Percent	100.00%	100.00%	100.00%	100.00%
T&D [Member] | Fixed-price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 157,357	$ 82,058	$ 410,253	$ 238,723
Percent	53.40%	36.90%	49.80%	37.50%
T&D [Member] | Unit Price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 49,392	$ 42,751	$ 152,748	$ 130,058
Percent	16.70%	19.20%	18.60%	20.50%
T&D [Member] | TE Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 84,299	$ 76,520	$ 237,448	$ 224,938
Percent	28.60%	34.40%	28.80%	35.40%
T&D [Member] | Other Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 3,892	$ 21,202	$ 22,949	$ 42,123
Percent	1.30%	9.50%	2.80%	6.60%
C&I [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 288,274	$ 177,006	$ 676,686	$ 448,982
Percent	100.00%	100.00%	100.00%	100.00%
C&I [Member] | Fixed-price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 220,872	$ 132,439	$ 469,700	$ 312,033
Percent	76.60%	74.80%	69.40%	69.50%
C&I [Member] | Unit Price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 19,518	$ 14,256	$ 42,361	$ 39,314
Percent	6.80%	8.10%	6.30%	8.80%
C&I [Member] | TE Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 32,868	$ 9,431	$ 82,592	$ 27,830
Percent	11.40%	5.30%	12.20%	6.20%
C&I [Member] | Other Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Amount	$ 15,016	$ 20,880	$ 82,033	$ 69,805
Percent	5.20%	11.80%	12.10%	15.50%