OTHER CURRENT ASSETS AND OTHER ASSETS - Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
OTHER CURRENT ASSETS AND OTHER ASSETS
Non-trade receivables	$ 7,582	$ 7,668
Prepaid rent	4,454	4,248
Prepaid maintenance	3,548	3,134
Restricted cash	2,311	3,271	$ 2,199	$ 2,255
Prepaid insurance	739	271
Prepaid benefits	621	368
Other	1,487	1,928
Total other current assets	$ 20,742	$ 20,888