License and Collaboration Agreements - Additional Information (Details)	1 Months Ended					3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2019 USD ($)	Oct. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)		Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
License And Collaboration Agreements [Line Items]
Deferred revenue, current						$ 6,250,000		$ 5,000,000	[1]	$ 6,250,000		$ 5,000,000	[1]
Deferred revenue, noncurrent						18,546,000		18,587,000	[1]	18,546,000		18,587,000	[1]
Taiho Pharmaceutical Co Ltd | Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period				5 years
Non refundable and non creditable cash payments				$ 35,000,000
Payment received for license agreement												25,000,000
Payment for option exercise										$ 3,000,000
Range of royalties receivable on net sales										high single-digits to mid-teens
Royalties payable description										Royalties will be payable on a licensed product-by-licensed product and country-by-country basis during the period of time commencing on the first commercial sale of a licensed product in a country and ending upon the later of: (a) ten (10) years from the date of first commercial sale of such licensed product in such country; and (b) expiration of the last-to-expire valid claim of the Company’s patents covering the manufacture, use or sale or exploitation of such licensed product in such country (the Royalty Term).
Non refundable and non creditable cash payments received										$ 5,000,000		$ 25,000,000
Estimated performance period										4 years		5 years
Remaining non refundable and non creditable cash payments receivable						5,000,000				$ 5,000,000
Payment for first option exercise			$ 3,000,000
Clinical and regulatory milestones achieved										0
Revenue recognized						4,300,000				6,800,000
Receivable from collaboration partner						5,000,000				5,000,000
Deferred revenue, current						6,300,000		5,000,000		6,300,000		$ 5,000,000
Deferred revenue, noncurrent						18,500,000		18,600,000		18,500,000		$ 18,600,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Subsequent Event
License And Collaboration Agreements [Line Items]
Additional payment to be received for license agreement		$ 5,000,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Scenario Forecast
License And Collaboration Agreements [Line Items]
Additional payment to be received for license agreement	$ 5,000,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Minimum
License And Collaboration Agreements [Line Items]
Number of programs, IND enabling studies not initiated | Program				5
Payment for option exercise				$ 3,000,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Maximum
License And Collaboration Agreements [Line Items]
Extended option agreement period				7 years
Payment for option exercise				$ 15,000,000
Additional clinical and regulatory milestone payments receivable				130,000,000
Contingent payments receivable				$ 145,000,000			$ 145,000,000				$ 145,000,000
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments								$ 18,500,000
Milestone payments						0				$ 0
Range of tiered royalty payments on net sales										high single-digits to low teens
WuXi Biologics License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization milestone payments					$ 375,000,000
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments						$ 2,000,000	$ 0			$ 2,800,000	$ 3,800,000
Abmuno License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization remaining milestone payments										$ 101,000,000

[1] The Condensed Consolidated Balance Sheet as of December 31, 2017 has been derived from the audited financial statements as of that date.