Revenue (Summary of Type of Customer, Contract Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Contract revenues	$ 620,795	$ 592,159	$ 637,468
Foreign [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	14,088	42,306	59,413
Type of Customer [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	620,795	592,159	637,468
Type of Customer [Member] | Federal Government [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	468,421	375,276	409,941
Type of Customer [Member] | State and Local Government [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	93,499	145,196	126,395
Type of Customer [Member] | Private [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	44,787	29,381	41,719
Type of Customer [Member] | Foreign [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	$ 14,088	$ 42,306	$ 59,413