Revenue (Summary of Type of Customer, Contract Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenues	$ 726,149	$ 733,601	$ 711,518
Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	6,596	25,892	48,619
Type of Customer [Member] | Operating Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenues	726,149	733,601	711,518
Type of Customer [Member] | Operating Segment [Member] | Federal Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	568,980	582,949	581,157
Type of Customer [Member] | Operating Segment [Member] | State and Local Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	118,712	85,737	71,398
Type of Customer [Member] | Operating Segment [Member] | Private [Member]
Disaggregation Of Revenue [Line Items]
Revenues	31,861	39,023	10,344
Type of Customer [Member] | Operating Segment [Member] | Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 6,596	$ 25,892	$ 48,619