Organization and Significant Accounting Policies - Additional Information (Detail)				1 Months Ended	12 Months Ended
	Oct. 07, 2020 USD ($)	Oct. 03, 2018 USD ($) Target	Sep. 28, 2016 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2018 shares	Sep. 30, 2018 USD ($)	Jul. 31, 2020 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock				$ 250,500,000	$ 250,478,974	$ 60,521,739		$ 56,585,035
Cash and cash equivalents					143,582,667	221,804,128
Restricted Cash					1,800,000	1,000,000.0
Short term investments					86,889,751	36,899,894
Marketable securities					85,000,000.0
Long term investments					137,486,883	44,175,993
Net increase (decrease) in cash and investments					150,100,000
Purchases of investments and marketable securities					278,964,290	90,266,001		52,083,131
Cash outflow primarily related to operations					95,391,570	(173,034,923)		$ 47,223,417
Maturities of investments					50,100,000
Fair value of contingent consideration obligation due to discontinuation of clinical trials					$ 0	$ 0
Calculation of effect of dilutive stock options and restricted stock units | shares					0	4,748,958	0
Right-of-use assets					$ 16,137,085
Right-of-use liabilities					$ 21,137,955
ASU No. 2014-09
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]					true
Change in accounting principle, accounting standards update, adoption date					Oct. 01, 2018
Change in accounting principle, accounting standards update, immaterial effect [true false]					true
ASU No. 2016-02
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]					true
Change in accounting principle, accounting standards update, adoption date					Oct. 01, 2019
Change in accounting principle, accounting standards update, immaterial effect [true false]					true
ASU No. 2018-18
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, immaterial effect [true false]					true
Change in accounting principle, accounting standards update, early adoption [true false]					true
Stock Options
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units | shares						1,007,500
Restricted Stock Units
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units | shares						11,500
Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement	$ 300,000,000.0
Profit sharing structure ratio	50.00%
Development regulatory and commercial milestones payments	$ 740,000,000.0
Janssen | JNJ-3989 (ARO-HBV) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment		$ 25,000,000.0			$ 25,000,000.0
Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Number of additional targets for development and sales milestone payments | Target		3
Janssen and JJDC | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement		$ 175,000,000.0
JJDC | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock		75,000,000.0
Amgen Incorporated | Olpasiran Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment									$ 20,000,000.0
Milestone payments			$ 30,000,000.0
Amgen Incorporated | License Collaboration and Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement			35,000,000.0
Proceeds from the issuance of common stock			$ 21,500,000
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account					$ 250,000
Property and equipment, useful life					7 years
Maximum | Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Tiered royalties received percentage	25.00%
Maximum | Janssen | License Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments		1,600,000,000
Maximum | Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments		$ 1,900,000,000
Maximum | Amgen Incorporated | Olpasiran Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments					$ 400,000,000.0
Minimum
Organization And Significant Accounting Policies [Line Items]
Property and equipment, useful life					3 years
Minimum | Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Tiered royalties received percentage	20.00%