SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently Issued Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Recently Issued Accounting Pronouncements
Net current deferred tax assets, noncurrent	$ 2,521
Provision for income taxes	4,198	$ 1,054	$ 3,189
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Recently Issued Accounting Pronouncements
Revenue, gross of tax	3,400
Increase to retained earnings offset	1,800
Cumulative catch-up adjustment to retained earnings	1,600
ASU 2015-17 | Adjustment
Recently Issued Accounting Pronouncements
Net current deferred tax assets, noncurrent	1,100
ASU 2016-09 | Adjustment
Recently Issued Accounting Pronouncements
Provision for income taxes	$ 300