REVENUES - Rollforward of Contract Assets (Details) - USD ($) $ in Thousands		9 Months Ended
	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Change in Contract with Customer, Asset and Liability [Roll Forward]
Balance at March 31, 2019, current		$ 8,844
Balance at March 31, 2019, noncurrent		16,104
Amounts recognized as expense in the period		(7,333)
Balance at June 30, 2019, current	$ 8,844	9,202
Balance at June 30, 2019, noncurrent	16,104	15,032
Balance at December 31, 2018		8,533
Revenue recognized that was included in the deferred revenue balance at December 31, 2018	(5,931)
Increases due to cash received, excluding amounts recognized as revenue during the period		5,150
Balance at September 30, 2019	8,533	7,752
Current
Change in Contract with Customer, Asset and Liability [Roll Forward]
Balance at March 31, 2019, current		8,844
Deferred customer acquisition costs incurred in the period		1,345
Transition between short-term and long-term		6,346
Balance at June 30, 2019, current	8,844	9,202
Non-current
Change in Contract with Customer, Asset and Liability [Roll Forward]
Balance at March 31, 2019, noncurrent		16,104
Deferred customer acquisition costs incurred in the period			$ 5,274
Amounts recognized as expense in the period			0
Transition between short-term and long-term			$ (6,346)
Balance at June 30, 2019, noncurrent	$ 16,104	$ 15,032