Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($)		12 Months Ended
	Nov. 18, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Organization And Significant Accounting Policies [Line Items]
Cash and cash equivalents		$ 24,838,567	$ 85,366,448	$ 81,214,354	$ 132,510,610
Short term investments		40,769,539
Net increase (decrease) in cash and investments		(19,800,000)
Cash outflows related to operating activities excluding upfront payment from Amgen		(53,900,000)
Capital expenditures		7,918,198	3,860,237	$ 1,970,612
Proceeds from the issuance of common stock		12,419,119	52,231,433
Restricted Cash		0	0
Fair value of contingent consideration obligation due to discontinuation of clinical trials		0	$ 0
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account		$ 250,000
Property, Plant and Equipment, Useful Life		7 years
Minimum
Organization And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life		3 years
Amgen
Organization And Significant Accounting Policies [Line Items]
Proceeds from upfront payments and equity investments		$ 42,500,000
Amgen | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock	$ 12,500,000
Amgen | ARO-LPA (AMG 890) Agreement
Organization And Significant Accounting Policies [Line Items]
Upfront payment	$ 30,000,000
Amgen | ARO-LPA (AMG 890) Agreement | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Common stock issued	1,745,810