Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (99,533)	$ (85,343)	$ (67,543)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,587	4,464	2,994
Loss on disposal of property and equipment	1	75	166
Equity-based compensation	15,091	17,046	15,322
Accretion of investment discounts	(3,725)	(676)
Changes in operating assets and liabilities:
Accounts receivable	2,927	2,924	(4,017)
Prepaid expenses and other current assets	(1,763)	310	(1,893)
Operating right-of-use assets	5,728
Other assets	153	1,022	902
Accounts payable	1,880	232	(488)
Accrued expenses	2,310	2,780	2,394
Deferred revenue	(27,122)	(3,936)	(12,988)
Operating lease liabilities	(4,774)
Other long-term liabilities		(155)	(125)
Net cash used in operating activities	(103,240)	(61,257)	(65,276)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(6,794)	(6,358)	(10,091)
Proceeds from sale of property and equipment		131
Purchases of marketable securities	(297,030)	(254,555)
Maturities of marketable securities	329,000
Net cash provided by (used in) investing activities	25,176	(260,782)	(10,091)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from common stock offerings, net of offering costs	72,256	28,547	141,000
Proceeds from options exercised	3,086	10,652	1,156
Issuance of shares through employee stock purchase plan	1,092	1,018	825
Net cash provided by financing activities	76,434	40,217	142,981
Net (decrease) increase in cash and cash equivalents	(1,630)	(281,822)	67,614
Cash and cash equivalents, beginning of period	58,856	340,678	273,064
Cash and cash equivalents, end of period	57,226	58,856	340,678
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	800	$ 1,071	$ 805
Right-of-use assets acquired under operating leases	$ 2,554