Condensed Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended		70 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014
Cash flows from operating activities
Net loss	$ (41,860)	$ (12,683)	$ (96,704)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	59	12	152
Loss on sale of assets	26		34
Reserve for uncollectible receivables			52
Stock-based compensation expense	3,049	144	3,600
Amortization of debt discount		121	666
Non-cash interest expense		7	195
Non-cash research and development expense			1
Remeasurement of convertible preferred stock call option liability	9,560	(863)	7,464
Remeasurement of convertible preferred stock warrant liability	2,279	(1)	2,239
Changes in assets and liabilities
Accounts receivable		84
Prepaid expenses and other assets	(2,030)	(75)	(3,350)
Accounts payable	(71)	(251)	244
Accrued liabilities and other liabilities	377	1,073	4,045
Net cash used in operating activities	(28,611)	(12,432)	(81,362)
Cash flows from investing activities
Proceeds from sale of property and equipment			10
Purchase of property and equipment	(600)	(9)	(785)
Security deposit for facility lease			(55)
Net cash used in investing activities	(600)	(9)	(830)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs	132,138		132,138
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	20,165	120,113
Proceeds from exercise of convertible preferred stock warrants	570		1,470
Proceeds from exercise of common stock options	4	27	53
Proceeds from issuance of convertible notes payable			9,000
Net cash provided by financing activities	197,505	20,192	263,774
Net increase in cash and cash equivalents	168,294	7,751	181,582
Cash and cash equivalents at beginning of period	13,288	329
Cash and cash equivalents at end of period	181,582	8,080	181,582
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		990	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		9,581
Conversion of preferred stock warrant liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	2,753		2,753
Conversion of preferred stock to common stock [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	$ 122,290		$ 122,290