Organization - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 104,946	$ 48,491
Cash and cash equivalents	$ 387,802	$ 293,579	$ 306,153	$ 176,677