Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Disaggregation of Revenue
Revenue	$ 648,787	$ 1,152,906
Fixed price
Disaggregation of Revenue
Revenue	200,564	403,446
Unit price
Disaggregation of Revenue
Revenue	291,662	475,250
Cost reimbursable
Disaggregation of Revenue
Revenue	156,561	274,210
MSA
Disaggregation of Revenue
Revenue	238,738	385,183
Non-MSA
Disaggregation of Revenue
Revenue	410,049	767,723
Power
Disaggregation of Revenue
Revenue	167,001	333,556
Power | Fixed price
Disaggregation of Revenue
Revenue	108,383	225,038
Power | Unit price
Disaggregation of Revenue
Revenue	14,532	25,644
Power | Cost reimbursable
Disaggregation of Revenue
Revenue	44,086	82,874
Power | MSA
Disaggregation of Revenue
Revenue	22,672	42,070
Power | Non-MSA
Disaggregation of Revenue
Revenue	144,329	291,486
Pipeline
Disaggregation of Revenue
Revenue	90,605	148,188
Pipeline | Fixed price
Disaggregation of Revenue
Revenue	28,102	40,622
Pipeline | Unit price
Disaggregation of Revenue
Revenue	31,678	50,323
Pipeline | Cost reimbursable
Disaggregation of Revenue
Revenue	30,825	57,243
Pipeline | MSA
Disaggregation of Revenue
Revenue	12,213	19,493
Pipeline | Non-MSA
Disaggregation of Revenue
Revenue	78,392	128,695
Utilities
Disaggregation of Revenue
Revenue	228,852	395,562
Utilities | Fixed price
Disaggregation of Revenue
Revenue	41,299	105,363
Utilities | Unit price
Disaggregation of Revenue
Revenue	127,863	194,614
Utilities | Cost reimbursable
Disaggregation of Revenue
Revenue	59,690	95,585
Utilities | MSA
Disaggregation of Revenue
Revenue	168,336	288,103
Utilities | Non-MSA
Disaggregation of Revenue
Revenue	60,516	107,459
Transmission
Disaggregation of Revenue
Revenue	42,454	42,454
Transmission | Fixed price
Disaggregation of Revenue
Revenue	8,000	8,000
Transmission | Unit price
Disaggregation of Revenue
Revenue	25,457	25,457
Transmission | Cost reimbursable
Disaggregation of Revenue
Revenue	8,997	8,997
Transmission | MSA
Disaggregation of Revenue
Revenue	35,517	35,517
Transmission | Non-MSA
Disaggregation of Revenue
Revenue	6,937	6,937
Civil
Disaggregation of Revenue
Revenue	119,875	233,146
Civil | Fixed price
Disaggregation of Revenue
Revenue	14,780	24,423
Civil | Unit price
Disaggregation of Revenue
Revenue	92,132	179,212
Civil | Cost reimbursable
Disaggregation of Revenue
Revenue	12,963	29,511
Civil | Non-MSA
Disaggregation of Revenue
Revenue	$ 119,875	$ 233,146