THE COMPANY AND SIGNIFICANT ACCOUNTING POLICIES - Adoption Impact of ASC 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	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	Jun. 30, 2016	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Apr. 01, 2018
Current assets:
Deferred sales commission costs	$ 15,601				$ 0								$ 15,601	$ 0		$ 11,234
Other current assets	15,127				10,040								15,127	10,040		11,765
Non-current assets:
Deferred sales commission costs, non-current	33,693				0								33,693	0		26,942
Stockholders' equity:
Accumulated deficit	(250,302)				(201,464)								(250,302)	(201,464)		(161,563)
Income Statement Related Disclosures [Abstract]
Total revenue	93,767	$ 89,912	$ 85,682	$ 83,225	79,344	$ 75,575	$ 72,483	$ 69,098	$ 66,489	$ 63,676	$ 63,183	$ 60,041	352,586	296,500	$ 253,388
Sales and marketing	49,525	46,276	41,680	40,495	40,422	36,382	28,680	28,460	27,351	25,770	24,328	21,444	177,976	133,945	98,893
Loss from operations	(28,780)	(24,238)	(21,987)	(15,983)	(13,730)	(18,247)	(4,462)	(5,457)	$ (3,685)	$ (1,703)	$ (379)	$ (901)	(90,988)	(41,896)	(6,669)
Net loss	$ (28,131)	$ (23,771)	$ (21,482)	$ (15,355)	$ (13,262)	$ (88,520)	$ (546)	$ (2,169)					$ (88,739)	$ (104,497)	$ (4,751)
Net loss per share:
Basic and diluted (in dollars per share)	$ (0.29)	$ (0.25)	$ (0.23)	$ (0.16)	$ (0.14)	$ (0.96)	$ (0.01)	$ (0.02)					$ (0.94)	$ (1.14)	$ (0.05)
Statement of Cash Flows [Abstract]
Net loss	$ (28,131)	$ (23,771)	$ (21,482)	$ (15,355)	$ (13,262)	$ (88,520)	$ (546)	$ (2,169)					$ (88,739)	$ (104,497)	$ (4,751)
Amortization of deferred sales commission costs													14,204	0	0
Deferred sales commission costs													(25,286)	0	0
Other current and non-current assets													(4,337)	(3,149)	(2,515)
Net cash provided by operating activities													(14,868)	22,041	28,478
Service
Income Statement Related Disclosures [Abstract]
Total revenue													334,438	280,430	235,816
Product
Income Statement Related Disclosures [Abstract]
Total revenue													18,148	16,070	$ 17,572
ASC 605
Current assets:
Deferred sales commission costs	0				0								0	0
Other current assets	9,410				10,040								9,410	10,040
Non-current assets:
Deferred sales commission costs, non-current	0				0								0	0
Stockholders' equity:
Accumulated deficit	(305,313)				$ (201,464)								(305,313)	$ (201,464)
Income Statement Related Disclosures [Abstract]
Total revenue													351,942
Sales and marketing													189,058
Loss from operations													(102,714)
Net loss													$ (100,465)
Net loss per share:
Basic and diluted (in dollars per share)													$ (1.06)
Statement of Cash Flows [Abstract]
Net loss													$ (100,465)
Amortization of deferred sales commission costs													0
Deferred sales commission costs													0
Other current and non-current assets													(3,693)
Net cash provided by operating activities													(14,868)
ASC 605 | Service
Income Statement Related Disclosures [Abstract]
Total revenue													335,671
ASC 605 | Product
Income Statement Related Disclosures [Abstract]
Total revenue													16,271
Adjustments | Accounting Standards Update 2014-09
Current assets:
Deferred sales commission costs	15,601												15,601			11,234
Other current assets	5,717												5,717			1,725
Non-current assets:
Deferred sales commission costs, non-current	33,693												33,693			26,942
Stockholders' equity:
Accumulated deficit	$ 55,011												55,011			$ 39,901
Income Statement Related Disclosures [Abstract]
Total revenue													644
Sales and marketing													(11,082)
Loss from operations													11,726
Net loss													$ 11,726
Net loss per share:
Basic and diluted (in dollars per share)													$ 0.12
Statement of Cash Flows [Abstract]
Net loss													$ 11,726
Deferred sales commission costs													(25,286)
Other current and non-current assets													(644)
Net cash provided by operating activities													0
Adjustments | Accounting Standards Update 2014-09 | Service
Income Statement Related Disclosures [Abstract]
Total revenue													(1,233)
Adjustments | Accounting Standards Update 2014-09 | Product
Income Statement Related Disclosures [Abstract]
Total revenue													$ 1,877