Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (36,060)	$ (32,630)	$ (21,884)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	395	202	248
Stock-based compensation expense	2,160	1,658	897
Accretion of discount/premium on investment securities	148
Non-cash interest expense	277	293	74
Gain on the sale of fixed assets	(52)
Changes in assets and liabilities:
Prepaid expenses and other current assets	(229)	(450)	(208)
Other assets		2
Accounts payable	77	196	481
Accrued expenses	418	954	8
Deferred rent	8	(18)	(28)
Net cash used in operating activities	(32,858)	(29,793)	(20,412)
Investing activities
Purchases of available-for-sale securities	(45,539)		(4,976)
Sales and maturities of available-for-sale securities	30,456		4,976
Purchases of property and equipment	(459)	(421)	(228)
Sale of property and equipment	52
Net cash used in investing activities	(15,490)	(421)	(228)
Financing activities
Proceeds from initial public offering, net of issuance costs		61,744
Proceeds from issuance of preferred stock, net of issuance costs		12,331
Proceeds from registered direct offering, net of issuance costs	10,603
Proceeds from at-the-market offering, net of issuance costs	1,391
Proceeds from exercise of common stock options and warrants	503	91	118
Proceeds from borrowing		5,000	5,000
Payments on borrowing	(3,333)	(833)
Debt issuance costs		(7)	(284)
Net cash provided by financing activities	9,164	78,326	4,834
Net (decrease) increase in cash and cash equivalents	(39,184)	48,112	(15,806)
Cash and cash equivalents, beginning of period	62,780	14,668	30,474
Cash and cash equivalents, end of period	23,596	62,780	14,668
Supplemental disclosure of cash flow information
Cash paid for interest	$ 582	684	100
Non-cash financing activities
Warrants for the purchase of series B preferred stock issued in conjunction with credit facility		107	110
Initial public offering costs in accounts payable and accrued liabilities		(492)	$ 492
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital		1,787
Reclassification of warrant liability to additional paid-in capital		$ 206