Revenue from Contracts with Customers Revenue by Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue by Contract Type [Line Items]
Revenue	$ 633,224	$ 610,912
Cost-reimbursable
Revenue by Contract Type [Line Items]
Revenue	419,775	418,655
Fixed-price
Revenue by Contract Type [Line Items]
Revenue	132,208	121,555
Time-and-materials
Revenue by Contract Type [Line Items]
Revenue	$ 81,241	$ 70,702