REVENUE - Affect of Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended												6 Months Ended		9 Months Ended		12 Months Ended			36 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	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2019	Jul. 01, 2018
ASSETS
Prepaid expenses and other current assets	$ 1,558				$ 929	$ 1,278	$ 1,610	$ 1,056	$ 879	$ 1,594	$ 1,677	$ 1,352	$ 1,610	$ 1,677	$ 1,278	$ 1,594	$ 1,558	$ 929	$ 879	$ 1,558	$ 1,180
Other assets	2,099				720	414	458	289	319	231	144	163	458	144	414	231	2,099	720	319	2,099	1,974
LIABILITIES
Deferred revenue	1,539				511	511	730	439	268	310	478	94	730	478	511	310	1,539	511	268	1,539	1,638
SHAREHOLDERS' EQUITY
Accumulated deficit	(264,400)				(232,748)	(231,052)	(227,829)	(223,635)	(221,531)	(218,593)	(217,668)	(217,436)	(227,829)	(217,668)	(231,052)	(218,593)	(264,400)	(232,748)	(221,531)	(264,400)	(232,372)
STATEMENT OF OPERATIONS
License and transaction fees	38,225	$ 37,646	$ 34,406	$ 33,522	42,125	33,592	31,532	25,259	31,779	26,301	21,787	21,569	56,791	43,356	90,383	69,657	143,799	132,508	101,436
Selling, general and administrative						9,629	9,005	6,924	8,134	6,542	6,029	7,472	15,929	13,501	25,558	20,043	47,068	34,647	28,177
Net loss	(10,541)	$ (4,510)	$ (10,657)	$ (6,320)	(1,696)	(3,223)	(4,194)	(2,171)	(2,938)	(925)	(232)	(3,370)	(6,365)	(3,602)	(9,588)	(4,527)	(32,028)	(11,284)	(7,465)	(50,800)
Calculated under Revenue Guidance in Effect before Topic 606
ASSETS
Prepaid expenses and other current assets	1,257				929												1,257	929		1,257
Other assets	593				720												593	720		593
LIABILITIES
Deferred revenue	610				511												610	511		610
SHAREHOLDERS' EQUITY
Accumulated deficit	(265,278)				$ (232,748)												(265,278)	(232,748)		(265,278)
STATEMENT OF OPERATIONS
Selling, general and administrative																	47,371
Net loss																	(32,528)
Adjustment | ASC 606
ASSETS
Prepaid expenses and other current assets	(301)																(301)			(301)	251
Other assets	(1,506)																(1,506)			(1,506)	1,254
LIABILITIES
Deferred revenue	(929)																(929)			(929)	1,127
SHAREHOLDERS' EQUITY
Accumulated deficit	$ (878)																(878)			$ (878)	$ 376
STATEMENT OF OPERATIONS
Selling, general and administrative																	303
Net loss																	(500)
Service
STATEMENT OF OPERATIONS
License and transaction fees						$ 25,381	$ 23,514	$ 19,397	$ 18,676	$ 17,458	$ 16,637	$ 16,363	$ 42,911	$ 33,000	$ 68,292	$ 50,458	123,554	$ 96,872	$ 69,134
Service | Calculated under Revenue Guidance in Effect before Topic 606
STATEMENT OF OPERATIONS
License and transaction fees																	123,356
Service | Adjustment | ASC 606
STATEMENT OF OPERATIONS
License and transaction fees																	$ (198)