Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue
Revenue	$ 908,216	$ 789,929	$ 1,651,459	$ 1,451,487
Fixed price
Disaggregation of Revenue
Revenue	204,234	182,586	422,013	328,871
Unit price
Disaggregation of Revenue
Revenue	497,117	333,374	824,003	592,574
Cost reimbursable
Disaggregation of Revenue
Revenue	206,865	273,969	405,443	530,042
MSA
Disaggregation of Revenue
Revenue	335,311	348,313	605,719	641,192
Non-MSA
Disaggregation of Revenue
Revenue	572,905	441,616	1,045,740	810,295
Power
Disaggregation of Revenue
Revenue	157,476	172,170	353,669	317,553
Power | Fixed price
Disaggregation of Revenue
Revenue	119,336	105,105	264,186	180,248
Power | Unit price
Disaggregation of Revenue
Revenue		4,042	181	10,655
Power | Cost reimbursable
Disaggregation of Revenue
Revenue	38,140	63,023	89,302	126,650
Power | MSA
Disaggregation of Revenue
Revenue	26,505	43,689	63,600	92,884
Power | Non-MSA
Disaggregation of Revenue
Revenue	130,971	128,481	290,069	224,669
Pipeline
Disaggregation of Revenue
Revenue	289,559	137,243	481,082	272,057
Pipeline | Fixed price
Disaggregation of Revenue
Revenue	24,006	14,109	39,351	31,336
Pipeline | Unit price
Disaggregation of Revenue
Revenue	150,331	10,130	232,161	10,504
Pipeline | Cost reimbursable
Disaggregation of Revenue
Revenue	115,222	113,004	209,570	230,217
Pipeline | MSA
Disaggregation of Revenue
Revenue	32,817	20,153	79,549	42,002
Pipeline | Non-MSA
Disaggregation of Revenue
Revenue	256,742	117,090	401,533	230,055
Utilities
Disaggregation of Revenue
Revenue	230,175	222,312	377,345	368,518
Utilities | Fixed price
Disaggregation of Revenue
Revenue	30,683	30,120	54,772	52,887
Utilities | Unit price
Disaggregation of Revenue
Revenue	155,006	119,618	235,645	187,496
Utilities | Cost reimbursable
Disaggregation of Revenue
Revenue	44,486	72,574	86,928	128,135
Utilities | MSA
Disaggregation of Revenue
Revenue	182,601	172,371	286,612	291,833
Utilities | Non-MSA
Disaggregation of Revenue
Revenue	47,574	49,941	90,733	76,685
Transmission
Disaggregation of Revenue
Revenue	109,948	135,354	212,732	253,797
Transmission | Fixed price
Disaggregation of Revenue
Revenue	12,843	14,251	26,479	22,714
Transmission | Unit price
Disaggregation of Revenue
Revenue	97,020	115,679	185,809	221,520
Transmission | Cost reimbursable
Disaggregation of Revenue
Revenue	85	5,424	444	9,563
Transmission | MSA
Disaggregation of Revenue
Revenue	92,656	110,875	174,430	212,598
Transmission | Non-MSA
Disaggregation of Revenue
Revenue	17,292	24,479	38,302	41,199
Civil
Disaggregation of Revenue
Revenue	121,058	122,850	226,631	239,562
Civil | Fixed price
Disaggregation of Revenue
Revenue	17,366	19,001	37,225	41,686
Civil | Unit price
Disaggregation of Revenue
Revenue	94,760	83,905	170,207	162,399
Civil | Cost reimbursable
Disaggregation of Revenue
Revenue	8,932	19,944	19,199	35,477
Civil | MSA
Disaggregation of Revenue
Revenue	732	1,225	1,528	1,875
Civil | Non-MSA
Disaggregation of Revenue
Revenue	$ 120,326	$ 121,625	$ 225,103	$ 237,687