Revenue from Contracts with Customers Revenue by Customer (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer [Line Items]
Revenue	$ 636,196	$ 579,179	$ 1,879,600	$ 1,618,146
U.S. Government
Revenue from External Customer [Line Items]
Revenue	624,888	567,496	1,848,043	1,583,489
State agencies, international agencies and commercial entities
Revenue from External Customer [Line Items]
Revenue	$ 11,308	$ 11,683	$ 31,557	$ 34,657