Acquisition (Details) - USD ($) $ in Thousands				1 Months Ended		12 Months Ended			22 Months Ended
	May 31, 2018	May 30, 2018	Aug. 24, 2016	Jun. 30, 2019	Aug. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense						$ 193,382	$ 143,183	$ 74,460
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment									$ 11,500
Upfront option fee					$ 500
Contingent consideration payment				$ 1,500
Contingent consideration recognized	$ 0					1,500
Maximum development plan period for research costs			24 months
Upfront purchase price less estimated net liabilities acquired							18,300
Research and development expense						$ 1,100	$ 800	$ 1,100
Exercise of buy-out option | Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial option exercise payments			$ 17,800
Exercise of buy-out option | Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma | Preclinical
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon achievement of milestones		$ 27,000
Exercise of buy-out option | Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma | Clinical
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon achievement of milestones		50,000
Exercise of buy-out option | Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma | Regulatory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon achievement of milestones		120,000
Exercise of buy-out option | Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma | Commercial
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon achievement of milestones		250,000
Maximum | Exercise of buy-out option | Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon achievement of milestones		$ 447,000