Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (43,575)	$ (28,224)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,085	1,371
(Gain) Loss on disposal of property and equipment	(29)	62
Equity-based compensation	9,008	5,479
Changes in operating assets and liabilities:
Accounts receivable	1,862	3,690
Prepaid expenses and other current assets	199	(320)
Accounts payable	(1,599)	(670)
Accrued expenses	305	(252)
Deferred revenue	(9,009)	(8,710)
Other assets	564	557
Other long-term liabilities	(76)	(59)
Net cash used in operating activities	(40,265)	(27,076)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(2,750)	(5,434)
Net cash used in investing activities	(2,750)	(5,434)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from options exercised	7,277	137
Issuance of shares through employee stock purchase plan	598	356
Net cash provided by financing activities	7,875	493
Net decrease in cash and cash equivalents	(35,140)	(32,017)
Cash and cash equivalents, beginning of period	340,678	273,064
Cash and cash equivalents, end of period	305,538	241,047
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	$ 1,417	$ 1,040