Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (15,160)	$ (13,573)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,807	1,031
Depreciation and amortization	580	590
Deferred rent costs	(214)	(194)
Changes in operating assets and liabilities:
Income tax receivable	(250)	(28)
Other receivables	(2,143)
Prepaid expenses	(1,649)	240
Accounts payable and accrued expenses	1,171	1,647
Deferred revenue	(1,452)	(1,531)
Net cash used in operating activities	(17,310)	(11,818)
Investing activities
Purchases of property and equipment	(13)	(27)
Net cash used in investing activities	(13)	(27)
Financing activities
Proceeds from initial public offering, net of issuance costs		57,762
Proceeds from follow-on offering of common stock, net of issuance costs	75,453
Proceeds from the exercise of stock options	323	119
Net cash provided by financing activities	75,776	57,881
Net cash increase for the period	58,453	46,036
Cash and cash equivalents at beginning of period	52,663	12,357
Cash and cash equivalents at end of period	111,116	58,393
Noncash financing activities
Conversion of convertible preferred stock to common stock		65,586
Reclassification of prepaid IPO costs paid in 2013		1,465
Unpaid IPO/ follow-on offering issuance costs	$ 222	$ 50