Condensed Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended		67 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014
Cash flows from operating activities
Net loss	$ (28,026)	$ (7,674)	$ (82,870)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	18	11	111
Loss on sale of assets	4		13
Reserve for uncollectible receivables			54
Stock-based compensation expense	1,493	88	2,041
Amortization of debt discount		121	666
Non-cash interest expense			195
Non-cash research and development expense			1
Remeasurement of convertible preferred stock call option liability	9,560	(612)	7,464
Remeasurement of convertible preferred stock warrant liability	2,279		2,239
Changes in assets and liabilities
Accounts receivable		(120)
Prepaid expenses and other assets	(4,026)	1,172	(5,345)
Accounts payable	781	(190)	1,096
Accrued liabilities and other liabilities	423	32	4,090
Net cash used in operating activities	(17,494)	(7,172)	(70,245)
Cash flows from investing activities
Proceeds from sale of property and equipment			10
Purchase of property and equipment	(483)	(9)	(668)
Security deposit for facility lease			(55)
Change in restricted cash			(75)
Net cash used in investing activities	(483)	(9)	(788)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs	132,095		132,095
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	7,703	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	197,462	7,730	263,731
Net increase in cash and cash equivalents	179,485	549	192,698
Cash and cash equivalents at beginning of period	13,288	329
Cash and cash equivalents at end of period	192,773	878	192,773
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			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
Initial public offering issuance costs	230		230
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	2,752		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		$ 122,290