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Revenue (Summary of Type of Customer, Contract Revenues) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disaggregation Of Revenue [Line Items]      
Revenues $ 733,601 $ 711,518 $ 620,795
Foreign [Member]      
Disaggregation Of Revenue [Line Items]      
Revenues 25,892 48,619 14,088
Type of Customer [Member] | Operating Segment [Member]      
Disaggregation Of Revenue [Line Items]      
Revenues 733,601 711,518 620,795
Type of Customer [Member] | Operating Segment [Member] | Federal Government [Member]      
Disaggregation Of Revenue [Line Items]      
Revenues 582,949 581,157 468,421
Type of Customer [Member] | Operating Segment [Member] | State and Local Government [Member]      
Disaggregation Of Revenue [Line Items]      
Revenues 85,737 71,398 93,499
Type of Customer [Member] | Operating Segment [Member] | Private [Member]      
Disaggregation Of Revenue [Line Items]      
Revenues 39,023 10,344 44,787
Type of Customer [Member] | Operating Segment [Member] | Foreign [Member]      
Disaggregation Of Revenue [Line Items]      
Revenues $ 25,892 $ 48,619 $ 14,088