Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net loss	$ (23,296)	$ (15,725)	$ (9,649)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,108	238	210
Depreciation expense	21	10	9
Release of reserve for loan to former advisor	(158)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,830)	(190)	139
Accounts payable	760	(759)	(119)
Accrued expenses	1,274	950	77
Net cash used in operating activities	(21,121)	(15,476)	(9,333)
Cash flows from investing activities:
Purchases of property and equipment	(25)	(23)	(8)
Change in restricted cash	(150)	(30)
Net cash used in investing activities	(175)	(53)	(8)
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions	97,929
Payments of initial public offering costs	(3,304)	(30)
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		35,406	18,775
Repayment of notes receivable	158
Proceeds from exercise of common stock options	16	215	4
Net cash provided by financing activities	94,799	35,591	18,779
Net increase in cash and cash equivalents	73,503	20,062	9,438
Cash and cash equivalents at beginning of period	31,753	11,691	2,253
Cash and cash equivalents at end of period	105,256	31,753	11,691
Supplemental disclosure of non-cash investing and financing activities:
Accretion of redeemable convertible preferred stock to redemption value		94	34
Conversion of redeemable convertible preferred stock to common stock	(85,345)
Conversion of convertible promissory notes and accrued interest and advance from stockholder to shares of redeemable convertible preferred stock			$ 141