Revenue from Contracts with Customers Revenue by Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue by Contract Type [Line Items]
Revenue	$ 648,578	$ 632,492	$ 1,281,802	$ 1,243,404
Cost-reimbursable
Revenue by Contract Type [Line Items]
Revenue	443,950	432,876	863,725	851,531
Fixed-price
Revenue by Contract Type [Line Items]
Revenue	117,474	120,359	249,682	241,914
Time-and-materials
Revenue by Contract Type [Line Items]
Revenue	$ 87,154	$ 79,257	$ 168,395	$ 149,959