Organization and Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	3 Months Ended
	Sep. 28, 2016 USD ($) $ / shares shares	Oct. 31, 2018 USD ($) Target $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Sep. 30, 2018 USD ($)	Aug. 01, 2018 USD ($)	Sep. 30, 2017 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock			$ 60,521,729
Cash and cash equivalents			189,772,981	$ 11,531,345	$ 30,133,213		$ 24,838,567
Short term investments			53,980,307		46,400,176
Long term investments			59,595,287
Net increase (decrease) in cash and investments			226,800,000
Promissory note payable			2,300,000
Restricted Cash			0		0
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			4,498,360	0
Stock Options and Restricted Stock Units
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options | shares			680,000
Collaboration and License agreements | Amgen
Organization And Significant Accounting Policies [Line Items]
Cash received as due under collaboration agreement	$ 35,000,000
Proceeds from the issuance of common stock	$ 21,500,000
Janssen and JJDC | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Cash received as due under collaboration agreement		$ 175,000,000
Proceeds from the issuance of common stock		$ 75,000,000
Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Number of additional targets for development and sales milestone payments | Target		3
Amgen | AMG-890 (ARO-LPA) Agreement
Organization And Significant Accounting Policies [Line Items]
Amgen milestone payment						$ 10,000,000
Common Stock Purchase Agreement | JJDC
Organization And Significant Accounting Policies [Line Items]
Shares issued | shares		3,260,869
Stock issued, price per share | $ / shares		$ 23.00
Common Stock Purchase Agreement | Janssen and JJDC
Organization And Significant Accounting Policies [Line Items]
Shares issued | shares		3,260,869
Stock issued, price per share | $ / shares		$ 23.00
Proceeds from the issuance of common stock		$ 75,000,000
Common Stock Purchase Agreement | Amgen
Organization And Significant Accounting Policies [Line Items]
Shares issued | shares	3,002,793
Stock issued, price per share | $ / shares	$ 7.16
Minimum
Organization And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life			3 years
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account			$ 250,000
Property, Plant and Equipment, Useful Life			7 years
Maximum | Collaboration and License agreements | Amgen
Organization And Significant Accounting Policies [Line Items]
Option payments, and development, regulatory and sales milestone payments.	$ 617,000,000
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