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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 2,999,816	$ 2,506,231	$ 5,703,397	$ 5,270,326
Percent of total revenues	100.00%	100.00%	100.00%	100.00%
Unit-price contracts
Disaggregation of Revenue [Line Items]
Revenues	$ 1,217,724	$ 918,416	$ 2,194,286	$ 1,893,067
Percent of total revenues	40.60%	36.60%	38.50%	36.00%
Cost-plus contracts
Disaggregation of Revenue [Line Items]
Revenues	$ 759,485	$ 567,928	$ 1,422,257	$ 1,256,012
Percent of total revenues	25.30%	22.70%	24.90%	23.80%
Fixed price contracts
Disaggregation of Revenue [Line Items]
Revenues	$ 1,022,607	$ 1,019,887	$ 2,086,854	$ 2,121,247
Percent of total revenues	34.10%	40.70%	36.60%	40.20%
United States
Disaggregation of Revenue [Line Items]
Revenues	$ 2,570,798	$ 2,207,876	$ 4,776,914	$ 4,475,962
Percent of total revenues	85.70%	88.10%	83.70%	85.00%
Canada
Disaggregation of Revenue [Line Items]
Revenues	$ 327,159	$ 212,820	$ 741,005	$ 597,045
Percent of total revenues	10.90%	8.50%	13.00%	11.30%
Australia
Disaggregation of Revenue [Line Items]
Revenues	$ 62,808	$ 56,077	$ 117,915	$ 107,127
Percent of total revenues	2.10%	2.20%	2.10%	2.00%
Others
Disaggregation of Revenue [Line Items]
Revenues	$ 39,051	$ 29,458	$ 67,563	$ 90,192
Percent of total revenues	1.30%	1.20%	1.20%	1.70%