Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net income (loss)	$ (16,926)	$ 21,901
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation	248	337
Net amortization of premiums and discounts on investments	(66)	4
Stock-based compensation	1,609	1,164
Other non-cash items	37	26
Changes in operating assets and liabilities:
Accounts receivable		(9)
Prepaid expenses and other current assets	(330)	389
Accounts payable	(2,693)	(3,444)
Accrued expenses	(3,128)	(4,463)
Operating lease assets	444
Operating lease liabilities	(430)
Deferred revenue	(11)	(40,592)
Net cash used in operating activities	(21,246)	(24,687)
Cash flows from investing activities
Maturities of marketable securities	19,500	10,500
Purchase of property and equipment	(65)	(372)
Net cash provided by investing activities	19,435	10,128
Cash flows from financing activities
Net proceeds from offering of common stock		92,162
Proceeds from exercise of stock options	119	42
Payments from issuance of debt	(180)
Payments under capital lease obligations	(29)
Net cash provided by (used in) financing activities	(90)	92,204
Increase (decrease) in cash, cash equivalents and restricted cash	(1,901)	77,645
Cash, cash equivalents and restricted cash, beginning of period	62,672	60,005
Cash, cash equivalents and restricted cash, end of period	60,771	137,650
Supplemental disclosures of non-cash activities:
Purchases of property and equipment in accounts payable and accrued expenses		26
Cash paid for interest	56
Right-of-use assets obtained in exchange for operating lease liabilities	$ 9,980	4,369
Right-of-use assets obtained in exchange for financing lease liabilities		$ 429