Revenue Recognition - Schedule of revenue by contract type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Contract revenues	$ 610,182	$ 607,901	$ 1,852,241	$ 1,639,422
Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	100.00%	100.00%	100.00%	100.00%
Fixed price
Disaggregation of Revenue [Line Items]
Contract revenues	$ 368,988	$ 375,165	$ 1,165,359	$ 1,028,880
Fixed price | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	60.50%	61.70%	62.90%	62.80%
Unit price
Disaggregation of Revenue [Line Items]
Contract revenues	$ 104,767	$ 111,295	$ 316,966	$ 296,433
Unit price | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	17.20%	18.30%	17.10%	18.10%
T&E
Disaggregation of Revenue [Line Items]
Contract revenues	$ 136,427	$ 121,441	$ 369,916	$ 314,109
T&E | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	22.30%	20.00%	20.00%	19.10%
T&D
Disaggregation of Revenue [Line Items]
Contract revenues	$ 306,546	$ 299,739	$ 948,285	$ 835,768
T&D | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	100.00%	100.00%	100.00%	100.00%
T&D | Fixed price
Disaggregation of Revenue [Line Items]
Contract revenues	$ 122,185	$ 126,588	$ 431,673	$ 374,065
T&D | Fixed price | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	39.90%	42.20%	45.50%	44.80%
T&D | Unit price
Disaggregation of Revenue [Line Items]
Contract revenues	$ 83,200	$ 88,708	$ 258,489	$ 236,720
T&D | Unit price | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	27.10%	29.60%	27.30%	28.30%
T&D | T&E
Disaggregation of Revenue [Line Items]
Contract revenues	$ 101,161	$ 84,443	$ 258,123	$ 224,983
T&D | T&E | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	33.00%	28.20%	27.20%	26.90%
C&I
Disaggregation of Revenue [Line Items]
Contract revenues	$ 303,636	$ 308,162	$ 903,956	$ 803,654
C&I | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	100.00%	100.00%	100.00%	100.00%
C&I | Fixed price
Disaggregation of Revenue [Line Items]
Contract revenues	$ 246,803	$ 248,577	$ 733,686	$ 654,815
C&I | Fixed price | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	81.30%	80.70%	81.20%	81.50%
C&I | Unit price
Disaggregation of Revenue [Line Items]
Contract revenues	$ 21,567	$ 22,587	$ 58,477	$ 59,713
C&I | Unit price | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	7.10%	7.30%	6.50%	7.40%
C&I | T&E
Disaggregation of Revenue [Line Items]
Contract revenues	$ 35,266	$ 36,998	$ 111,793	$ 89,126
C&I | T&E | Product concentration risk | Revenue benchmark
Disaggregation of Revenue [Line Items]
Concentration risk, Percent	11.60%	12.00%	12.30%	11.10%