License and Collaboration Agreements - Additional Information (Details)	1 Months Ended				3 Months Ended		6 Months Ended	12 Months Ended
	Oct. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Mar. 31, 2019 USD ($) shares	Mar. 31, 2018 USD ($) 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					69	708,760
Revenue recognized					$ 1,750,000
Deferred revenue, current					7,000,000				$ 6,250,000	[1]
Deferred revenue, noncurrent					$ 12,272,000				16,984,000	[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		$ 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
Deferred revenue, current					7,000,000
Deferred revenue, noncurrent					12,300,000
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
Milestone payable					$ 7,500,000
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					$ 0	$ 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, 2018 has been derived from the audited financial statements as of that date.