Revenue (Summary of Contract Revenues by Type of Customer) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenues	$ 217,695	$ 192,637
Type of Customer [Member] | Operating Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenues	217,695	192,637
Type of Customer [Member] | Operating Segment [Member] | Federal Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	165,767	170,155
Type of Customer [Member] | Operating Segment [Member] | State and Local Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	42,332	14,103
Type of Customer [Member] | Operating Segment [Member] | Private [Member]
Disaggregation Of Revenue [Line Items]
Revenues	2,734	50
Type of Customer [Member] | Operating Segment [Member] | Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 6,862	$ 8,329