Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities
Net loss	$ (22,006)	$ (19,426)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	54	5
Stock-based compensation expense	1,988	426
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	(1,456)	(127)
Accounts payable	(657)	665
Accrued liabilities and other liabilities	1,553	810
Net cash used in operating activities	(20,524)	(5,808)
Cash flows from investing activities
Purchase of property and equipment	(5)	(38)
Net cash used in investing activities	(5)	(38)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs		132,188
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 exercise of common stock options	16	4
Net cash provided by financing activities	80,224	197,555
Net increase in cash and cash equivalents	59,695	191,709
Cash and cash equivalents at beginning of period	170,566	13,288
Cash and cash equivalents at end of period	230,261	204,997
Supplemental disclosure of noncash items
Offering issuance costs		1,293
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