Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets
Cash and cash equivalents	$ 170,566	$ 13,288
Prepaid expenses and other current assets	2,398	978
Total current assets	172,964	14,266
Other assets	616	396
Property and equipment, net	714	21
Total assets	174,294	14,683
Current liabilities
Accounts payable	1,259	315
Accrued liabilities	5,666	3,668
Total current liabilities	6,925	3,983
Convertible preferred stock warrant liability		474
Convertible preferred stock call option liability		21
Total liabilities	6,925	4,478
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.0001 par value; 5,000,000, and 135,816,462 shares authorized at December 31, 2014 and December 31, 2013, respectively; zero and 120,648,174 shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively; zero and $60,392 liquidation preference at December 31, 2014 and December 31, 2013, respectively		57,497
Stockholders' equity (deficit)
Common stock, $0.0001 par value, 50,000,000 and 15,652,174 shares authorized at December 31, 2014 and December 31, 2013, respectively; 24,245,437 and 1,257,311 shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively	2
Additional paid-in capital	278,626	6,454
Deficit accumulated during the development stage	(111,259)	(53,746)
Total stockholders' equity (deficit)	167,369	(47,292)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 174,294	$ 14,683