Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Detail) (USD $)	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 43,750	$ 43,750	$ 43,750	$ 43,750	$ 43,750	$ 43,750	$ 43,750	$ 159,016	$ 175,000	$ 290,266	$ 146,875
Operating Losses	(22,354,306)	(12,700,100)	(11,212,498)	(7,009,382)	(7,999,102)	(6,389,865)	(5,390,288)	(4,810,021)	(53,276,286)	(24,589,276)	(21,089,211)
Net loss	(22,430,889)	(11,626,451)	(13,982,700)	(10,685,372)	(13,805,059)	(6,177,628)	(6,940,019)	(4,780,727)	(58,725,412)	(31,703,433)	(22,110,723)
Net Loss Attributable to Arrowhead	$ (22,432,438)	$ (11,626,919)	$ (13,942,521)	$ (10,628,312)	$ (13,692,183)	$ (6,079,010)	$ (6,757,937)	$ (4,614,159)	$ (58,630,190)	$ (31,143,289)	$ (21,125,928)
Loss per share (Basic and Diluted)	$ (0.42)	$ (0.22)	$ (0.31)	$ (0.28)	$ (0.43)	$ (0.23)	$ (0.41)	$ (0.33)	$ (1.25)	$ (1.30)	$ (1.90)