Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue
Revenue	$ 743,243	$ 661,558
Fixed price
Disaggregation of Revenue
Revenue	217,779	146,285
Unit price
Disaggregation of Revenue
Revenue	326,886	259,200
Cost reimbursable
Disaggregation of Revenue
Revenue	198,578	256,073
MSA
Disaggregation of Revenue
Revenue	270,408	292,879
Non-MSA
Disaggregation of Revenue
Revenue	472,835	368,679
Power
Disaggregation of Revenue
Revenue	196,193	145,383
Power | Fixed price
Disaggregation of Revenue
Revenue	144,850	75,143
Power | Unit price
Disaggregation of Revenue
Revenue	181	6,613
Power | Cost reimbursable
Disaggregation of Revenue
Revenue	51,162	63,627
Power | MSA
Disaggregation of Revenue
Revenue	37,095	49,195
Power | Non-MSA
Disaggregation of Revenue
Revenue	159,098	96,188
Pipeline
Disaggregation of Revenue
Revenue	191,523	134,814
Pipeline | Fixed price
Disaggregation of Revenue
Revenue	15,345	17,227
Pipeline | Unit price
Disaggregation of Revenue
Revenue	81,830	374
Pipeline | Cost reimbursable
Disaggregation of Revenue
Revenue	94,348	117,213
Pipeline | MSA
Disaggregation of Revenue
Revenue	46,732	21,849
Pipeline | Non-MSA
Disaggregation of Revenue
Revenue	144,791	112,965
Utilities
Disaggregation of Revenue
Revenue	147,170	146,206
Utilities | Fixed price
Disaggregation of Revenue
Revenue	24,089	22,767
Utilities | Unit price
Disaggregation of Revenue
Revenue	80,639	67,878
Utilities | Cost reimbursable
Disaggregation of Revenue
Revenue	42,442	55,561
Utilities | MSA
Disaggregation of Revenue
Revenue	104,011	119,462
Utilities | Non-MSA
Disaggregation of Revenue
Revenue	43,159	26,744
Transmission
Disaggregation of Revenue
Revenue	102,784	118,443
Transmission | Fixed price
Disaggregation of Revenue
Revenue	13,636	8,463
Transmission | Unit price
Disaggregation of Revenue
Revenue	88,789	105,841
Transmission | Cost reimbursable
Disaggregation of Revenue
Revenue	359	4,139
Transmission | MSA
Disaggregation of Revenue
Revenue	81,774	101,723
Transmission | Non-MSA
Disaggregation of Revenue
Revenue	21,010	16,720
Civil
Disaggregation of Revenue
Revenue	105,573	116,712
Civil | Fixed price
Disaggregation of Revenue
Revenue	19,859	22,685
Civil | Unit price
Disaggregation of Revenue
Revenue	75,447	78,494
Civil | Cost reimbursable
Disaggregation of Revenue
Revenue	10,267	15,533
Civil | MSA
Disaggregation of Revenue
Revenue	796	650
Civil | Non-MSA
Disaggregation of Revenue
Revenue	$ 104,777	$ 116,062