Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 31,250	$ 39,583	$ 43,750	$ 43,750	$ 43,750	$ 123,750	$ 43,750	$ 170,750	$ 158,333	$ 382,000	$ 175,000
Operating Losses	(21,795,114)	(19,341,487)	(21,264,855)	(19,341,270)	(25,232,251)	(15,993,706)	(29,632,743)	(25,115,276)	(81,742,726)	(95,973,976)	(53,276,286)
Net loss	(22,221,985)	(19,420,743)	(20,815,860)	(19,264,414)	(24,745,554)	(15,936,053)	(28,683,993)	(22,575,282)	(81,723,002)	(91,940,882)	(58,725,412)
Net Loss Attributable to Arrowhead	$ (22,221,985)	$ (19,420,743)	$ (20,815,860)	$ (19,264,414)	$ (24,745,554)	$ (15,936,053)	$ (28,683,993)	$ (22,575,282)	$ (81,723,002)	$ (91,940,882)	$ (58,630,190)
Loss per share (Basic and Diluted)	$ (0.34)	$ (0.32)	$ (0.35)	$ (0.32)	$ (0.42)	$ (0.27)	$ (0.51)	$ (0.41)	$ (1.34)	$ (1.60)	$ (1.25)