CONTRACT BALANCES (Schedule of Changes in Allowance for Doubtful Accounts) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts:
Balance, beginning of period	$ 8,602	$ 6,196	$ 3,208
Add: provision for doubtful accounts	24,515	20,917	18,907
Less: write-offs, net of recoveries	(24,432)	(18,511)	(15,919)
Balance, end of period	$ 8,685	$ 8,602	$ 6,196