Summary of Significant Accounting Policies - Revenue Disaggregated by Geographic Location and Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 3,112,800	$ 3,352,895	$ 2,839,199	$ 2,807,259	$ 3,112,218	$ 2,985,281	$ 2,656,348	$ 2,417,576	$ 12,112,153	$ 11,171,423	$ 9,466,478
Percentage of total revenues									100.00%	100.00%
Unit-price contracts
Disaggregation of Revenue [Line Items]
Revenues									$ 4,193,295	$ 3,828,997
Percentage of total revenues									34.60%	39.40%
Cost-plus contracts
Disaggregation of Revenue [Line Items]
Revenues									$ 3,304,161	$ 2,507,025
Percentage of total revenues									27.30%	20.70%
Fixed price contracts
Disaggregation of Revenue [Line Items]
Revenues									$ 4,614,697	$ 4,835,401
Percentage of total revenues									38.10%	39.90%
United States
Disaggregation of Revenue [Line Items]
Revenues									$ 10,190,684	$ 8,575,341
Percentage of total revenues									84.00%	78.60%
Canada
Disaggregation of Revenue [Line Items]
Revenues									$ 1,436,720	$ 1,984,214
Percentage of total revenues									11.90%	16.40%
Australia
Disaggregation of Revenue [Line Items]
Revenues									$ 187,915	$ 377,453
Percentage of total revenues									1.60%	3.10%
Latin America and Other
Disaggregation of Revenue [Line Items]
Revenues									$ 296,834	$ 234,415
Percentage of total revenues									2.50%	1.90%