Contracts with Customers - Summary of Disaggregation of Revenue (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	$ 879,356	$ 979,459	$ 1,754,053	$ 1,950,452
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenues	233,231	311,368	463,173	619,676
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenues	253,365	259,381	493,030	511,820
Cost-Plus
Disaggregation Of Revenue [Line Items]
Revenues	$ 392,760	$ 408,710	$ 797,850	$ 818,956