License and Collaboration Agreements - Additional Information (Details)		1 Months Ended						3 Months Ended		6 Months Ended			12 Months Ended
	Apr. 30, 2019 USD ($) shares	Jun. 30, 2019 USD ($) shares	Oct. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)		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								18,701	5,171	18,770	713,931
Revenue recognized								$ 1,750,000		$ 3,500,000
Deferred revenue, current		$ 7,000,000						7,000,000		7,000,000				$ 6,250,000	[1]
Deferred revenue, noncurrent		10,522,000						10,522,000		10,522,000				16,984,000	[1]
Compensation expense recognized								2,130,000	$ 1,116,000	3,804,000	$ 1,772,000
Research and Development Expenses
License And Collaboration Agreements [Line Items]
Compensation expense recognized								868,000	701,000	$ 1,715,000	1,041,000
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		$ 25,000,000
Additional payment to be received for license agreement			$ 5,000,000
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		3,500,000
Deferred revenue, current		7,000,000						7,000,000		7,000,000
Deferred revenue, noncurrent		10,500,000						10,500,000		10,500,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Scenario Forecast
License And Collaboration Agreements [Line Items]
Payment received for license agreement													$ 5,000,000
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
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments												$ 18,500,000
Milestone payments								0	0	$ 0	0
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]
Upfront and milestone payments		$ 7,500,000
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									$ 0		$ 800,000
Milestone payments								$ 0		$ 0
Abmuno License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Upfront and milestone payments				$ 2,000,000
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		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.