Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (11,962)	$ (41,825)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	963	907
Loss on disposal of fixed assets		20
Net amortization of premiums and discounts on investments	(87)	(288)
Stock-based compensation	3,610	2,756
Non-cash rent expense		77
Non-cash interest expense	65
Changes in operating assets and liabilities:
Accounts receivable	161	198
Prepaid expenses and other current assets	1,318	(1,379)
Other assets		(1,433)
Accounts payable	(3,102)	3,619
Accrued expenses	(4,518)	3,568
Operating lease assets and liabilities	(66)
Deferred revenue	(41,550)	(5,532)
Net cash used in operating activities	(55,168)	(39,312)
Cash flows from investing activities
Maturities of marketable securities	10,500	74,565
Purchase of marketable securities	(53,688)
Purchase of property and equipment	(605)	(1,093)
Net cash provided by (used in) investing activities	(43,793)	73,472
Cash flows from financing activities
Net proceeds from public offering of common stock	92,162
Proceeds from exercise of stock options	121	853
Proceeds from purchases of common stock under ESPP	283
Proceeds from issuance of debt, net of issuance costs	4,965
Payments under capital lease obligations	(58)
Net cash provided by financing activities	97,473	853
Increase (decrease) in cash, cash equivalents and restricted cash	(1,488)	35,013
Cash, cash equivalents and restricted cash, beginning of period	60,005	26,962
Cash, cash equivalents and restricted cash, end of period	58,517	61,975
Supplemental disclosures of non-cash activities:
Purchases of property and equipment included in accounts payable and accrued expenses		75
Debt financing costs in accrued expenses	180
Cash paid for interest	64
Property and equipment acquired under finance leases	$ 429
Adjustment to accumulated deficit and deferred revenue upon adoption of Topic 606		$ 2,031