Revenue from Contracts with Customers Revenue by Customer (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue from External Customer [Line Items]
Revenue	$ 648,578	$ 632,492	$ 1,281,802	$ 1,243,404
U.S. Government
Revenue from External Customer [Line Items]
Revenue	642,755	622,627	1,271,253	1,223,155
State agencies, international agencies and commercial entities
Revenue from External Customer [Line Items]
Revenue	$ 5,823	$ 9,865	$ 10,549	$ 20,249