Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (52,023)	$ (47,667)	$ (32,718)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,328	1,087	799
Stock-based compensation	4,029	2,510	948
Revaluation of future purchase rights liabilities	0	(2,600)	1,306
Other	0	(7)	(1)
Changes in operating assets and liabilities:
Other assets and prepaid expenses	143	279	(1,141)
Accounts payable	281	46	(91)
Accrued expenses	(3,584)	5,396	2,266
Other liabilities	(126)	131	(16)
Net cash used in operating activities	(49,952)	(40,825)	(28,648)
Cash flows from investing activities:
Purchases of short-term investments	0	0	(2,999)
Sales of short-term investments	0	0	17,000
Change in restricted cash	1,059	(631)	(608)
Purchases of property and equipment	(2,340)	(1,409)	(1,484)
Net cash (used in) provided by investing activities	(1,281)	(2,040)	11,909
Cash flows from financing activities:
Deferred financing costs	(283)	0	(3,112)
Proceeds from issuance of common stock, net of issuance costs	32,189	87,899	0
Proceeds from issuance of preferred stock, net of issuance costs	0	18,167	53,195
Proceeds from exercise of stock options	507	852	135
Proceeds from early exercise of stock options	8	0	227
Net cash provided by financing activities	32,421	106,918	50,445
Effect of exchange rate changes on cash and cash equivalents	(10)	(1)	3
Net change in cash and cash equivalents	(18,822)	64,052	33,709
Cash and cash equivalents, beginning of period	102,238	38,186	4,477
Cash and cash equivalents, end of period	83,416	102,238	38,186
Supplemental disclosure of non-cash investing and financing activities:
Offering costs included in liabilities	39	0	394
Purchase of property and equipment included in liabilities	9	277	170
Leasehold improvements paid for by landlord	0	0	478
Change in stock option early exercise repurchase liability	108	104	0
Conversion of redeemable convertible preferred stock to common stock	0	110,796	0
Valuation of future purchase rights upon issuance	0	0	1,294
Beneficial conversion underlying the senior preferred stock	0	0	969
Accretion to redemption value of redeemable convertible preferred stock	0	4,410	6,303
Amortization of deemed dividend	$ 0	$ 4,744	$ 64