Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (24,690)	$ (17,745)	$ (3,963)
Adjustments to reconcile net loss to net cash (used in) provided by operations:
Stock-based compensation expense	2,514	1,371	123
Change in fair value of liability under license agreement			(35)
Accrued interest and amortization of beneficial conversion feature on promissory notes			3,605
Loss on write-off of fixed assets	2
Depreciation & amortization	839	783	789
Amortization/accretion of available-for-sale securities	(18)
Deferred rent costs	(289)	(265)	(238)
Amortization of financing costs			117
Changes in operating assets and liabilities:
Income tax receivable	(184)	(139)	(30)
Interest receivable	(80)
Prepaid expenses	(1,442)	388	(1,736)
Accounts payable and accrued expenses	3,322	(12)	722
Deferred revenue	(1,452)	(2,023)	3,475
Net cash (used in) provided by operating activities	(21,478)	(17,642)	2,829
Investing activities
Purchase of available-for-sale marketable securities	(91,657)
Purchases of property and equipment	(20)	(42)	(5)
Net cash used in investing activities	(91,677)	(42)	(5)
Financing activities
Proceeds from convertible promissory notes			1,462
Financing costs on convertible promissory notes			(70)
Repayment of convertible promissory notes			(311)
Repayment of long-term debt			(307)
Proceeds from sale of common stock		100
Proceeds from the exercise of stock options	362	128
Proceeds from initial public offering, net of issuance costs		57,762
Net cash provided by financing activities	75,593	57,990	8,416
Net cash increase (decrease) for the period	(37,562)	40,306	11,240
Cash and cash equivalents at beginning of period	52,663	12,357	1,117
Cash and cash equivalents at end of period	15,101	52,663	12,357
Supplemental disclosure of cash flow information
Cash paid for interest			37
Noncash financing activities
Conversion of convertible preferred stock to common stock		65,586
Reclassification of prepaid IPO costs paid in 2013		$ 1,465
Conversion of convertible promissory notes to Series D convertible preferred stock			3,888
Junior A Convertible Preferred Stock [Member]
Financing activities
Proceeds from sale of convertible preferred stock			$ 7,642
Follow-on Offering [Member]
Financing activities
Proceeds from follow-on offering, net of issuance costs	$ 75,231