Organization and Basis of Presentation - Additional Information (Detail) - USD ($)			9 Months Ended		12 Months Ended
	May. 11, 2015	Apr. 17, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Nov. 30, 2014	Dec. 31, 2013
Organization And Basis Of Presentation [Line Items]
Collaboration and license agreement, date			May 28, 2013
Accumulated deficit			$ (114,873,000)		$ (91,750,000)
Proceeds from issuance of common stock			12,675,000	$ 51,505,000
Cash and cash equivalents			$ 46,362,000	$ 35,943,000	54,059,000		$ 7,942,000
Silicon Valley Bank [Member]
Organization And Basis Of Presentation [Line Items]
Maximum borrowing capacity						$ 15,000,000
Amount borrowed	$ 5,000,000				$ 10,000,000.0
Initial Public Offering ("IPO") [Member]
Organization And Basis Of Presentation [Line Items]
Issuance of common stock		1,251,784
Proceeds from issuance of common stock		$ 12,200,000