REVENUE FROM CONTRACTS FROM CUSTOMERS Revenue by Customer Type and Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 126,690	$ 380,572
Department of Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	96,909	290,175
Department of Defense [Member] | Cost Reimbursement Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	44,080	131,641
Department of Defense [Member] | Time-and-materials Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	23,174	74,842
Department of Defense [Member] | Fixed-price Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	29,655	83,692
Non-Department of Defense U.S. Government [Member]
Disaggregation of Revenue [Line Items]
Revenue	23,745	71,900
Non-Department of Defense U.S. Government [Member] | Cost Reimbursement Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,219	6,093
Non-Department of Defense U.S. Government [Member] | Time-and-materials Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	19,062	57,956
Non-Department of Defense U.S. Government [Member] | Fixed-price Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,464	7,851
Commercial and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	6,036	18,497
Commercial and Other [Member] | Cost Reimbursement Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	0	0
Commercial and Other [Member] | Time-and-materials Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,078	2,906
Commercial and Other [Member] | Fixed-price Contract [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 4,958	$ 15,591