SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Restatement to Cash Flow of Prior Periods (Details) - USD ($) $ in Thousands	3 Months Ended											6 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net loss			$ (19,985)				$ (13,651)				$ (8,046)	$ (37,947)	$ (27,430)	$ (26,940)	$ (62,009)	$ (42,245)	$ (37,556)	$ (138,169)	$ (61,075)	$ (44,166)
Depreciation and amortization	$ 20,999	$ 21,505	22,176	$ 22,710	$ 23,009	$ 22,179	20,523	$ 16,793	$ 20,320	$ 18,755	17,561	43,321	42,702	36,316	64,320	65,710	56,636	85,713	88,416	73,429
Amortization of debt discount and issuance costs							1,058						2,132	1,082		3,218	1,680	5,398	4,084	2,309
Non-cash change in finance lease liabilities			(128)				(1,425)					3,370	(1,191)		4,506	(1,015)	913	3,300	2,507	1,187
Accounts receivable			(1,186)				1,243					2,345	(2,077)		2,728	(4,858)		3,272	(359)	(769)
Prepaid expenses, deposits and other assets							(1,011)						(4,509)			(3,967)	1,400	4,576	(1,232)	2,051
Accrued and current other liabilities			(2,563)				(2,821)					(3,471)	(751)		(5,703)	(952)		(4,288)	(1,916)	3,359
Deferred revenues			644				1,323					(10)	2,710		(573)	2,078		(1,496)	1,220	(1,297)
Accounts payable																	(1,589)	(3,076)	1,546	(949)
Stock-based compensation expense, net of capitalized amount			519									1,530	2,328			3,669		4,239	4,678	3,040
Net cash provided by operating activities							4,135						15,497			24,943		22,663	35,341	41,404
Net (decrease) increase in cash and cash equivalents							2,118						698			(2,197)		(6,174)	3,782	3,652
Cash and cash equivalents				17,823				14,041										$ 11,649	17,823	14,041	$ 10,389
As reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net loss			(19,622)				(14,261)				(8,230)	(38,157)	(28,517)	(27,513)	(62,006)	(43,971)	(38,377)		(62,375)	(45,273)
Depreciation and amortization	$ 21,582	$ 21,955	22,178	$ 23,254	$ 23,553	$ 22,712	21,158	17,397	$ 20,917	$ 18,934	17,745	44,133	43,870	36,679	65,715	67,422	57,596		90,676	74,993
Amortization of debt discount and issuance costs							638						1,712	1,292		2,798	1,890		3,874	2,519
Non-cash change in finance lease liabilities			148				(213)					3,520	(371)		4,863	(241)	564		2,640	520
Accounts receivable			(1,617)				864					1,210	(2,165)		1,593	(4,990)			(1,352)	(207)
Prepaid expenses, deposits and other assets							(467)						(4,073)			(3,531)	1,979
Accrued and current other liabilities			(2,238)				(2,904)					(3,146)	(585)		(5,378)	(601)			(1,583)
Deferred revenues			(262)				(138)					(323)	1,249		(804)	617			435
Accounts payable																	(2,168)
Stock-based compensation expense, net of capitalized amount			890									1,901	2,232			3,573
Net cash provided by operating activities							3,573						14,935			24,381			34,779	41,966
Net (decrease) increase in cash and cash equivalents							1,556						136			(2,759)			3,220	4,214
Cash and cash equivalents								14,603												14,603
Cash Flow Adjustment | Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net loss			(363)				610				184	210	1,087	573	(3)	1,726	821		1,300	1,107
Depreciation and amortization			(2)				(635)				$ (184)	(812)	(1,168)	(363)	(1,395)	(1,712)	(960)		(2,260)	(1,564)
Amortization of debt discount and issuance costs							420						420	$ (210)		420	(210)		210	(210)
Non-cash change in finance lease liabilities			(276)				(1,212)					(150)	(820)		(357)	(774)	349		(133)	667
Accounts receivable			431				379					1,135	88		1,135	132			993	(562)
Prepaid expenses, deposits and other assets							(544)						(436)			(436)	(579)
Accrued and current other liabilities			(325)				83					(325)	(166)		(325)	(351)			(333)
Deferred revenues			906				1,461					313	1,461		$ 231	1,461			785
Accounts payable																	$ 579
Stock-based compensation expense, net of capitalized amount			$ (371)									$ (371)	96			96
Net cash provided by operating activities							562						562			562			562	(562)
Net (decrease) increase in cash and cash equivalents							$ 562						$ 562			$ 562			$ 562	(562)
Cash and cash equivalents								$ (562)												$ (562)