License and Collaboration Agreements - Additional Information (Details)		1 Months Ended					3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Apr. 30, 2019 USD ($) shares	Oct. 31, 2019 USD ($)	Oct. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)
License And Collaboration Agreements [Line Items]
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares | shares							1,052	0		18,770	713,931
Revenue recognized							$ 1,750,000			$ 5,250,000
Deferred revenue, current							7,000,000			7,000,000		$ 6,250,000	[1]
Deferred revenue, noncurrent							13,772,000			13,772,000		16,984,000	[1]
Compensation expense recognized							2,740,000	$ 1,048,000		6,544,000	$ 2,820,000
Research and Development Expenses
License And Collaboration Agreements [Line Items]
Compensation expense recognized							978,000	638,000		2,693,000	1,679,000
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			$ 5,000,000	$ 5,000,000						$ 5,000,000		5,000,000		$ 25,000,000
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).
Upfront cash payment					$ 20,000,000
Non-refundable, non-creditable upfront cash payments										$ 35,000,000
Estimated performance period										5 years
Payment for first option exercise												$ 3,000,000
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares | shares										0
Clinical and regulatory milestones achieved										$ 0
Sales milestone or royalty revenue recognized										0
Revenue recognized							1,800,000			5,300,000
Deferred revenue, current							7,000,000			7,000,000
Deferred revenue, noncurrent							13,700,000			$ 13,700,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Subsequent Event
License And Collaboration Agreements [Line Items]
Payment 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
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments									$ 18,500,000
Range of tiered royalty payments on net sales										high single-digits to low teens
WuXi Biologics License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Milestone payments							7,500,000	0		$ 7,500,000	0
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]
Milestone payments							$ 0			0
Abmuno License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Upfront and milestone payments								$ 2,000,000			$ 2,800,000
Abmuno License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization remaining milestone payments										101,000,000
Strata Oncology Inc | Co-Development and Collaboration Agreement
License And Collaboration Agreements [Line Items]
Development milestone payable	$ 2,500,000
Development cost recorded within research and development expenses										$ 300,000
Number of restricted shares of common stock issued | shares	1,257,651
Fair value of restricted shares of common stock issued	$ 15,000,000
Strata Oncology Inc | Co-Development and Collaboration Agreement | Restricted Shares
License And Collaboration Agreements [Line Items]
Shares probable of vesting | shares							0			0
Compensation expense recognized										$ 0
Strata Oncology Inc | Co-Development and Collaboration Agreement | Maximum
License And Collaboration Agreements [Line Items]
Regulatory and commercial milestone payable	$ 125,000,000

[1] The Condensed Consolidated Balance Sheet as of December 31, 2018 has been derived from the audited financial statements as of that date.