Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 8,713,785	$ 9,342,498	$ 8,985,930	$ 4,365,496	$ 31,250	$ 39,583	$ 43,750	$ 43,750	$ 31,407,709	$ 158,333	$ 382,000
Operating Losses	(10,250,216)	(5,714,164)	(6,129,642)	(14,901,887)	(21,795,114)	(19,341,487)	(21,264,855)	(19,341,270)	(36,995,909)	(81,742,726)	(95,973,976)
Net loss	(10,731,889)	(5,519,741)	(6,042,557)	(12,086,108)	(22,221,985)	(19,420,743)	(20,815,860)	(19,264,414)	$ (34,380,295)	$ (81,723,002)	$ (91,940,882)
Net Loss Attributable to Arrowhead	$ (10,731,889)	$ (5,519,741)	$ (6,042,557)	$ (12,086,108)	$ (22,221,985)	$ (19,420,743)	$ (20,815,860)	$ (19,264,414)
Loss per share (Basic and Diluted)	$ (0.14)	$ (0.07)	$ (0.08)	$ (0.17)	$ (0.34)	$ (0.32)	$ (0.35)	$ (0.32)	$ (0.47)	$ (1.34)	$ (1.60)