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Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($)		6 Months Ended
	Nov. 18, 2016	Mar. 31, 2017	Mar. 31, 2016	Nov. 29, 2016	Sep. 30, 2016	Sep. 30, 2015
Schedule Of Investments [Line Items]
Percentage of workforce reduced due to discontinued clinical program				30.00%
Cash and cash equivalents		$ 61,718,971	$ 50,300,847		$ 85,366,448	$ 81,214,354
Short term investments		24,910,492
Net increase (decrease) in cash		(23,647,477)	(30,913,507)
Cash outflows related to operating activities excluding upfront payment from Amgen		(34,300,000)
Cash outflow from investments in commercial bonds		24,846,105
Capital expenditures		6,653,158	500,634
Proceeds from the issuance of common stock		12,691,937	$ 25,544
Restricted Cash		0			0
Short-term investment, gross unrealized losses		130,319
Fair value		24,780,173
Investments					0
Fair value of contingent consideration obligation due to discontinuation of clinical trials		0			$ 0
Corporate Bonds
Schedule Of Investments [Line Items]
Short term investments		24,910,492
Short-term investment, gross unrealized losses		130,319
Short-term investment, gross unrealized gain		0
Maximum
Schedule Of Investments [Line Items]
Amount insured in FDIC per account		$ 250,000
Property, Plant and Equipment, Useful Life		7 years
Minimum
Schedule Of Investments [Line Items]
Property, Plant and Equipment, Useful Life		3 years
Amgen
Schedule Of Investments [Line Items]
Proceeds from upfront payments and equity investments		$ 42,500,000
Amgen | Common Stock Purchase Agreement
Schedule Of Investments [Line Items]
Proceeds from the issuance of common stock	$ 12,500,000
Amgen | ARO-LPA Agreement
Schedule Of Investments [Line Items]
Common stock issued	1,745,810
Upfront payment	$ 30,000,000
Amgen | ARO-LPA Agreement | Common Stock Purchase Agreement
Schedule Of Investments [Line Items]
Common stock issued	1,745,810

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