Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended		62 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (47,623)	$ (43,575)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,621	2,085
Loss (gain) on disposal of property and equipment	17	(29)
Equity-based compensation	8,996	9,008
Accretion of investment discounts	(2,630)
Non-cash lease expense	462
Changes in operating assets and liabilities:
Accounts receivable	3,359	1,862
Prepaid expenses and other current assets	(1,339)	199
Accounts payable	(293)	(1,599)
Accrued expenses	947	305
Deferred revenue	(13,772)	(9,009)
Other assets	125	564
Other long-term liabilities		(76)
Net cash used in operating activities	(49,130)	(40,265)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(2,474)	(2,750)
Purchases of marketable securities	(182,582)
Maturities of marketable securities	214,000
Net cash provided by (used in) investing activities	28,944	(2,750)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from common stock offerings, net of offering costs	7,912		$ 580,200
Proceeds from options exercised	2,024	7,277
Issuance of shares through employee stock purchase plan	534	598
Net cash provided by financing activities	10,470	7,875
Net decrease in cash and cash equivalents	(9,716)	(35,140)
Cash and cash equivalents, beginning of period	58,856	340,678
Cash and cash equivalents, end of period	49,140	305,538	$ 49,140
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	867	$ 1,417
Proceeds from at-the-market offerings unpaid at period end	27,140
Right-of-use assets acquired under operating leases	$ 1,343