Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities
Net loss	$ (62,052)	$ (41,860)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	174	59
Loss on sale of assets		26
Stock-based compensation expense	8,491	3,049
Remeasurement of convertible preferred stock call option liability		9,560
Remeasurement of convertible preferred stock warrant liability		2,279
Changes in assets and liabilities
Prepaid expenses and other assets	7	(2,030)
Accounts payable	(119)	(71)
Accrued liabilities and other liabilities	582	377
Net cash used in operating activities	(52,917)	(28,611)
Cash flows from investing activities
Purchase of property and equipment	(4)	(600)
Net cash used in investing activities	(4)	(600)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs		132,138
Proceeds from issuance of common stock in follow-on offering, net of issuance costs	80,208
Proceeds from issuance of convertible preferred stock, net of issuance costs		64,793
Proceeds from exercise of convertible preferred stock warrants		570
Proceeds from issuance of equity awards in connection with equity incentive plans	161	4
Net cash provided by financing activities	80,369	197,505
Net increase in cash and cash equivalents	27,448	168,294
Cash and cash equivalents at beginning of period	170,566	13,288
Cash and cash equivalents at end of period	$ 198,014	181,582
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 to common stock and additional paid in capital		$ 122,290