NATURE OF OPERATIONS AND BASIS OF PRESENTATION - Restatement to Cash Flow of Prior Periods (Details) - USD ($) $ in Thousands	3 Months Ended										9 Months Ended
	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	Sep. 30, 2019	Sep. 30, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accounts payable	$ 2,431	$ 207	$ 1,546	$ 5,155	$ 3,341	$ (760)	$ (949)	$ (2,168)	$ (1,623)	$ (2,759)	$ 756	$ 5,155
Net cash provided by operating activities	13,137	1,706	34,779	24,381	14,935	3,573	41,966	29,270	22,534	6,752	18,659	24,381
Purchases of property and equipment	(14,698)	(7,538)	(38,505)	(23,100)	(12,504)	(5,958)	(35,932)	(24,528)	(10,193)	(5,277)	(23,277)	(23,100)
Net cash used in investing activities	(15,415)	(8,012)	(174,244)	(156,406)	(145,051)	(137,941)	(32,427)	(21,119)	(10,637)	(5,477)	(23,886)	(156,406)
Additions to property and equipment included in accounts payable	4,019	3,731	4,266	10,235	9,636	4,425	3,946	1,602	5,600	3,991	$ 5,153	$ 10,235
As reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accounts payable	3,375	763	1,339	9,372	6,939	(636)	(1,167)	(3,498)	477	(2,247)
Net cash provided by operating activities	14,081	2,262	34,572	28,598	18,533	3,697	41,748	27,940	24,634	7,264
Purchases of property and equipment	(15,642)	(8,094)	(38,298)	(27,317)	(16,102)	(6,082)	(35,714)	(23,198)	(12,293)	(5,789)
Net cash used in investing activities	(16,359)	(8,568)	(174,037)	(160,623)	(148,649)	(138,065)	(32,209)	(19,789)	(12,737)	(5,989)
Additions to property and equipment included in accounts payable	1,268	1,850	2,459	4,004	4,023	2,287	1,932	701	1,269	1,247
Cash Flow Adjustment | Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accounts payable	(944)	(556)	207	(4,217)	(3,598)	(124)	218	1,330	(2,100)	(512)
Net cash provided by operating activities	(944)	(556)	207	(4,217)	(3,598)	(124)	218	1,330	(2,100)	(512)
Purchases of property and equipment	944	556	(207)	4,217	3,598	124	(218)	(1,330)	2,100	512
Net cash used in investing activities	944	556	(207)	4,217	3,598	124	(218)	(1,330)	2,100	512
Additions to property and equipment included in accounts payable	$ 2,751	$ 1,881	$ 1,807	$ 6,231	$ 5,613	$ 2,138	$ 2,014	$ 901	$ 4,331	$ 2,744