Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue
Revenue	$ 942,700	$ 865,064	$ 2,594,159	$ 2,316,551
Fixed price
Disaggregation of Revenue
Revenue	262,394	214,353	684,407	543,224
Unit price
Disaggregation of Revenue
Revenue	382,124	380,541	1,206,127	973,115
Cost reimbursable
Disaggregation of Revenue
Revenue	298,182	270,170	703,625	800,212
MSA
Disaggregation of Revenue
Revenue	384,735	366,891	990,454	1,008,083
Non-MSA
Disaggregation of Revenue
Revenue	557,965	498,173	1,603,705	1,308,468
Power
Disaggregation of Revenue
Revenue	212,557	200,657	566,226	518,210
Power | Fixed price
Disaggregation of Revenue
Revenue	66,939	136,040	331,125	316,288
Power | Unit price
Disaggregation of Revenue
Revenue	1,993	2,954	2,174	13,609
Power | Cost reimbursable
Disaggregation of Revenue
Revenue	143,625	61,663	232,927	188,313
Power | MSA
Disaggregation of Revenue
Revenue	35,297	43,680	98,897	136,564
Power | Non-MSA
Disaggregation of Revenue
Revenue	177,260	156,977	467,329	381,646
Pipeline
Disaggregation of Revenue
Revenue	214,380	133,590	695,462	405,647
Pipeline | Fixed price
Disaggregation of Revenue
Revenue	152,301	13,860	191,652	45,196
Pipeline | Unit price
Disaggregation of Revenue
Revenue	45,627	21,949	277,788	32,453
Pipeline | Cost reimbursable
Disaggregation of Revenue
Revenue	16,452	97,781	226,022	327,998
Pipeline | MSA
Disaggregation of Revenue
Revenue	29,403	29,110	108,952	71,112
Pipeline | Non-MSA
Disaggregation of Revenue
Revenue	184,977	104,480	586,510	334,535
Utilities
Disaggregation of Revenue
Revenue	298,984	281,561	676,329	650,079
Utilities | Fixed price
Disaggregation of Revenue
Revenue	23,117	31,462	77,889	84,349
Utilities | Unit price
Disaggregation of Revenue
Revenue	185,791	165,183	421,436	352,679
Utilities | Cost reimbursable
Disaggregation of Revenue
Revenue	90,076	84,916	177,004	213,051
Utilities | MSA
Disaggregation of Revenue
Revenue	230,099	189,606	516,711	481,439
Utilities | Non-MSA
Disaggregation of Revenue
Revenue	68,885	91,955	159,618	168,640
Transmission
Disaggregation of Revenue
Revenue	114,221	128,784	326,953	382,581
Transmission | Fixed price
Disaggregation of Revenue
Revenue	7,480	13,034	33,959	35,748
Transmission | Unit price
Disaggregation of Revenue
Revenue	68,952	110,869	254,761	332,389
Transmission | Cost reimbursable
Disaggregation of Revenue
Revenue	37,789	4,881	38,233	14,444
Transmission | MSA
Disaggregation of Revenue
Revenue	89,312	103,421	263,742	316,019
Transmission | Non-MSA
Disaggregation of Revenue
Revenue	24,909	25,363	63,211	66,562
Civil
Disaggregation of Revenue
Revenue	102,558	120,472	329,189	360,034
Civil | Fixed price
Disaggregation of Revenue
Revenue	12,557	19,957	49,782	61,643
Civil | Unit price
Disaggregation of Revenue
Revenue	79,761	79,586	249,968	241,985
Civil | Cost reimbursable
Disaggregation of Revenue
Revenue	10,240	20,929	29,439	56,406
Civil | MSA
Disaggregation of Revenue
Revenue	624	1,074	2,152	2,949
Civil | Non-MSA
Disaggregation of Revenue
Revenue	$ 101,934	$ 119,398	$ 327,037	$ 357,085