Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended			73 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (57,513)	$ (18,497)	$ (13,217)	$ (112,357)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	107	17	21	200
Loss on disposition of assets	26			35
Reserve for uncollectible receivables				54
Stock-based compensation expense	4,641	215	141	5,189
Amortization of debt discount		121	312	666
Non-cash interest expense		7	81	195
Non-cash research and development expense				1
Remeasurement of convertible preferred stock call option liability	9,560	(969)	(89)	7,463
Remeasurement of convertible preferred stock warrant liability	2,279	41		2,239
Changes in assets and liabilities
Accounts receivable		84	491
Prepaid expenses and other assets	(1,695)	299	(128)	(3,013)
Accounts payable	944	(675)	413	1,259
Accrued liabilities and other liabilities	1,998	2,267	259	5,665
Net cash used in operating activities	(39,653)	(17,090)	(11,716)	(92,404)
Cash flows from investing activities
Proceeds from sale of property and equipment				10
Purchase of property and equipment	(827)	(9)		(1,012)
Security deposit for facility lease	55
Net cash used in investing activities	(772)	(9)		(1,002)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs	132,137			132,137
Proceeds from sale of option for Series A preferred stock purchase rights				1,000
Proceeds from issuance of convertible preferred stock, net of issuance costs	64,793	29,956	5,765	120,113
Proceeds from exercise of convertible preferred stock warrants	572		900	1,472
Proceeds from issuance of common stock in connection with employee benefit plans	201	27	5	250
Proceeds from issuance of convertible notes payable			4,500	9,000
Net cash provided by financing activities	197,703	29,983	11,170	263,972
Net increase (decrease) in cash and cash equivalents	157,278	12,884	(546)	170,566
Cash and cash equivalents at beginning of period	13,288	404	950
Cash and cash equivalents at end of period	170,566	13,288	404	170,566
Supplemental disclosure
Cash paid for interest				2
Supplemental disclosure of noncash items
Conversion of notes payable and accrued interest to preferred stock		4,588		9,195
Issuance of warrants for preferred stock in connection with convertible notes			433	666
Accretion of Series A convertible preferred stock to redemption value				11,002
Issuance of call options related to convertible preferred stock		1,116		3,504
Extinguishment of Series A convertible preferred stock				12,100
Conversion of preferred stock call option liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	9,581
Conversion of preferred stock warrant liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	2,752
Conversion of preferred stock to common stock [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	$ 122,290