Revenue Recognition - Schedule of revenue by contract type (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Contract revenues	$ 571,075	$ 583,214	$ 448,776	$ 468,094	$ 446,345	$ 399,537	$ 339,676	$ 345,611	$ 2,071,159	$ 1,531,169	$ 1,403,317
Concentration risk									100.00%	100.00%
Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 2,071,159	$ 1,531,169
Concentration risk									100.00%	100.00%
Fixed-price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 1,268,994	$ 814,431
Concentration risk									61.30%	53.20%
Unit Price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 282,656	$ 232,769
Concentration risk									13.60%	15.20%
TE Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 418,713	$ 340,519
Concentration risk									20.20%	22.20%
Other Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 100,796	$ 143,450
Concentration risk									4.90%	9.40%
T&D [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 1,134,411	$ 893,108
Concentration risk									100.00%	100.00%
T&D [Member] | Fixed-price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 564,251	$ 361,699
Concentration risk									49.70%	40.50%
T&D [Member] | Unit Price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 228,223	$ 181,179
Concentration risk									20.10%	20.30%
T&D [Member] | TE Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 316,943	$ 305,581
Concentration risk									27.90%	34.20%
T&D [Member] | Other Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 24,994	$ 44,649
Concentration risk									2.30%	5.00%
C&I [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 936,748	$ 638,061
Concentration risk									100.00%	100.00%
C&I [Member] | Fixed-price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 704,743	$ 452,732
Concentration risk									75.20%	71.00%
C&I [Member] | Unit Price Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 54,433	$ 51,590
Concentration risk									5.80%	8.10%
C&I [Member] | TE Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 101,770	$ 34,938
Concentration risk									10.90%	5.40%
C&I [Member] | Other Contract [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Contract revenues									$ 75,802	$ 98,801
Concentration risk									8.10%	15.50%