Revenue from Contracts with Customers Revenue by Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue by Contract Type [Line Items]
Revenue	$ 636,196	$ 579,179	$ 1,879,600	$ 1,618,146
Cost-reimbursable
Revenue by Contract Type [Line Items]
Revenue	431,991	397,921	1,283,522	1,129,538
Fixed-price
Revenue by Contract Type [Line Items]
Revenue	126,402	120,632	368,316	323,723
Time-and-materials
Revenue by Contract Type [Line Items]
Revenue	$ 77,803	$ 60,626	$ 227,762	$ 164,885