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Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2012	Sep. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted and outstanding to purchase	1,914,794		1,910,794	729,096
Options granted	(4,000)		1,229,500
Stock-based compensation expense	$ 395,674	$ 251,878
Fair value of the options granted	6,023	1,392,584
Intrinsic value of the options exercised	0	0
Arrowhead [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax compensation expense	3,356,689
Weighted average period to recognize pre-tax compensation expense	2 years 10 months 24 days
Calando Pharmaceuticals, Inc. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of the options granted	0	33,690
Pre-tax compensation expense	$ 53,624
Weighted average period to recognize pre-tax compensation expense	2 years 6 months
2000 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserve for issuance	153,200
Options granted and outstanding to purchase	152,900
2004 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserve for issuance	1,965,860
Options granted and outstanding to purchase	1,510,694
Options granted outside of Equity Incentive plans	251,200
Options granted	4,000

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Weighted average period to recognize pre-tax compensation expense.

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Aggregate number of common shares reserved for future issuance.

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Net number of non-option equity instruments granted to participants.

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The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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Gross number of share options (or share units) granted during the period.

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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

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