License and Acquisition Agreements - MedImmune License Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
License and Acquisition Agreements
Research and development		$ 20,901	$ 59,253
MedImmune | License Agreement
License and Acquisition Agreements
Upfront payment for exchange of rights	$ 8,000
Milestone payment to be paid upon regulatory milestone achievement	10,000
Accrued milestone payments	10,000				$ 5,000
Payment of accrued milestones				$ 10,000
Pass-through payment paid upon clinical milestone achievement				$ 5,000
Annual net sales excluded from calculation of specified annual net sales thresholds	$ 1,000,000
Maximum percentage of royalty payable on annual net sales of licensed products	20.00%
Notice period to terminate the agreement by both parties for material breaches	90 days
Notice period to terminate the agreement	90 days
Research and development		$ 0	$ 0
Minimum | MedImmune | License Agreement
License and Acquisition Agreements
Additional specified annual net sales threshold for additional milestone payment.	$ 1,000,000
Maximum | MedImmune | License Agreement
License and Acquisition Agreements
Milestone payment to be paid upon specified milestone achievements for first two indications	72,500
Milestone payment to be paid upon clinical and regulatory milestone achievement	15,000
Milestone payment to be paid upon specified annual sales milestone achievements	85,000
Additional milestone payments payable after achievement of additional annual sales	$ 1,100,000