OTHER CURRENT ASSETS AND OTHER ASSETS - Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
OTHER CURRENT ASSETS AND OTHER ASSETS
Non-trade receivables	$ 7,668				$ 6,223
Prepaid rent	4,248				4,070
Prepaid maintenance	3,134				3,914
Restricted cash	3,271				2,255
Prepaid other	1,436				1,542
Prepaid education and training	582				398
Prepaid insurance	271				447
Other	278				288
Total other current assets	$ 20,888	$ 19,176	$ 19,066	$ 18,465	$ 19,137	$ 18,102	$ 19,199	$ 24,014