Summary of Significant Accounting Policies - Revenue Disaggregated by Geographic Location 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 [Line Items]
Revenues	$ 2,506,231	$ 2,839,199	$ 5,270,326	$ 5,646,458
Percent of total revenues	100.00%	100.00%	100.00%	100.00%
Unit-price contracts
Disaggregation of Revenue [Line Items]
Revenues	$ 918,416	$ 1,020,650	$ 1,893,067	$ 1,915,694
Percent of total revenues	36.60%	35.90%	36.00%	33.90%
Cost-plus contracts
Disaggregation of Revenue [Line Items]
Revenues	$ 567,928	$ 1,103,135	$ 1,256,012	$ 2,061,490
Percent of total revenues	22.70%	38.90%	23.80%	36.50%
Fixed price contracts
Disaggregation of Revenue [Line Items]
Revenues	$ 1,019,887	$ 715,414	$ 2,121,247	$ 1,669,274
Percent of total revenues	40.70%	25.20%	40.20%	29.60%
United States
Disaggregation of Revenue [Line Items]
Revenues	$ 2,207,876	$ 2,561,924	$ 4,475,962	$ 4,762,539
Percent of total revenues	88.10%	90.30%	85.00%	84.30%
Canada
Disaggregation of Revenue [Line Items]
Revenues	$ 212,820	$ 202,221	$ 597,045	$ 687,651
Percent of total revenues	8.50%	7.10%	11.30%	12.20%
Australia
Disaggregation of Revenue [Line Items]
Revenues	$ 56,077	$ 36,886	$ 107,127	$ 78,210
Percent of total revenues	2.20%	1.30%	2.00%	1.40%
Latin America and Other
Disaggregation of Revenue [Line Items]
Revenues	$ 29,458	$ 38,168	$ 90,192	$ 118,058
Percent of total revenues	1.20%	1.30%	1.70%	2.10%