Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities
Net loss	$ (12,403)	$ (8,061)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	258	209
Loss on disposal of fixed assets	20
Net amortization of premiums and discounts on investments	(157)
Stock-based compensation	745	264
Change in deferred rent	13	(28)
Changes in operating assets and liabilities:
Accounts receivable	496	339
Prepaid expenses and other current assets	(1,086)	(509)
Other assets	(1,433)	(1,025)
Accounts payable	(617)	(53)
Accrued expenses	(1,249)	589
Deferred revenue	(1,977)	(2,635)
Net cash used in operating activities	(17,390)	(10,910)
Cash flows from investing activities
Maturities of marketable securities	23,165
Purchase of property and equipment	(267)	(471)
Net cash provided by (used in) investing activities	22,898	(471)
Cash flows from financing activities
Proceeds from exercise of stock options	255	104
Payments of initial public offering costs		(505)
Net cash provided by (used in) financing activities	255	(401)
Increase (decrease) in cash, cash equivalents and restricted cash	5,763	(11,782)
Cash, cash equivalents and restricted cash, beginning of period	26,962	100,668
Cash, cash equivalents and restricted cash, end of period	32,725	88,886
Supplemental disclosures of non-cash activities:
Purchases of property and equipment included in accounts payable and accrued expenses	320	$ 154
ASU 2014-09
Supplemental disclosures of non-cash activities:
Adjustment to accumulated deficit and deferred revenue upon adoption of Topic 606	2,031
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Cash flows from operating activities
Net loss	$ (897)