Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 43,750	$ 123,750	$ 43,750	$ 170,750	$ 43,750	$ 43,750	$ 43,750	$ 43,750	$ 382,000	$ 175,000	$ 290,266
Operating Losses	(25,232,251)	(15,993,706)	(29,632,743)	(25,115,276)	(22,354,306)	(12,700,100)	(11,212,498)	(7,009,382)	(95,973,976)	(53,276,286)	(24,589,276)
Net loss	(24,745,554)	(15,936,053)	(28,683,993)	(22,575,282)	(22,430,889)	(11,626,451)	(13,982,700)	(10,685,372)	(91,940,882)	(58,725,412)	(31,703,433)
Net Loss Attributable to Arrowhead	$ (24,745,554)	$ (15,936,053)	$ (28,683,993)	$ (22,575,282)	$ (22,432,438)	$ (11,626,919)	$ (13,942,521)	$ (10,628,312)	$ (91,940,882)	$ (58,630,190)	$ (31,143,289)
Loss per share (Basic and Diluted)	$ (0.42)	$ (0.27)	$ (0.51)	$ (0.41)	$ (0.42)	$ (0.22)	$ (0.31)	$ (0.28)	$ (1.60)	$ (1.25)	$ (1.30)