Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Disaggregation of Revenue
Revenue	$ 908,902	$ 2,061,808
Fixed price
Disaggregation of Revenue
Revenue	211,735	615,181
Unit price
Disaggregation of Revenue
Revenue	337,970	813,220
Cost reimbursable
Disaggregation of Revenue
Revenue	359,197	633,407
MSA
Disaggregation of Revenue
Revenue	390,409	775,592
Non-MSA
Disaggregation of Revenue
Revenue	518,493	1,286,216
Power
Disaggregation of Revenue
Revenue	181,822	515,378
Power | Fixed price
Disaggregation of Revenue
Revenue	85,561	310,599
Power | Unit price
Disaggregation of Revenue
Revenue	10,371	36,015
Power | Cost reimbursable
Disaggregation of Revenue
Revenue	85,890	168,764
Power | MSA
Disaggregation of Revenue
Revenue	48,004	90,074
Power | Non-MSA
Disaggregation of Revenue
Revenue	133,818	425,304
Pipeline
Disaggregation of Revenue
Revenue	213,073	361,261
Pipeline | Fixed price
Disaggregation of Revenue
Revenue	41,772	82,394
Pipeline | Unit price
Disaggregation of Revenue
Revenue	7,924	58,247
Pipeline | Cost reimbursable
Disaggregation of Revenue
Revenue	163,377	220,620
Pipeline | MSA
Disaggregation of Revenue
Revenue	14,986	34,479
Pipeline | Non-MSA
Disaggregation of Revenue
Revenue	198,087	326,782
Utilities
Disaggregation of Revenue
Revenue	269,652	665,214
Utilities | Fixed price
Disaggregation of Revenue
Revenue	42,763	148,126
Utilities | Unit price
Disaggregation of Revenue
Revenue	144,611	339,225
Utilities | Cost reimbursable
Disaggregation of Revenue
Revenue	82,278	177,863
Utilities | MSA
Disaggregation of Revenue
Revenue	227,192	515,295
Utilities | Non-MSA
Disaggregation of Revenue
Revenue	42,460	149,919
Transmission
Disaggregation of Revenue
Revenue	121,526	163,980
Transmission | Fixed price
Disaggregation of Revenue
Revenue	20,259	28,259
Transmission | Unit price
Disaggregation of Revenue
Revenue	84,646	110,103
Transmission | Cost reimbursable
Disaggregation of Revenue
Revenue	16,621	25,618
Transmission | MSA
Disaggregation of Revenue
Revenue	100,227	135,744
Transmission | Non-MSA
Disaggregation of Revenue
Revenue	21,299	28,236
Civil
Disaggregation of Revenue
Revenue	122,829	355,975
Civil | Fixed price
Disaggregation of Revenue
Revenue	21,380	45,803
Civil | Unit price
Disaggregation of Revenue
Revenue	90,418	269,630
Civil | Cost reimbursable
Disaggregation of Revenue
Revenue	11,031	40,542
Civil | Non-MSA
Disaggregation of Revenue
Revenue	$ 122,829	$ 355,975