Collaboration and Other Agreements - Incyte (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Oct. 31, 2017 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) performance_obligation
Collaboration And Other Agreements [Line Items]
Revenues		$ 9,662	$ 4,695
Incyte Corporation | Incyte MGA012 Agreement
Collaboration And Other Agreements [Line Items]
Non-refundable upfront payment				$ 150,000
Potential development and regulatory milestone payments under agreement	$ 420,000
Potential commercial milestone payments under agreement	330,000
Development and regulatory milestones recognized		15,000
Number of performance obligations | performance_obligation				2
Collaborative agreement transaction price	154,000
Variable consideration recognized	$ 4,000
Maximum | Incyte Corporation | Incyte MGA012 Agreement
Collaboration And Other Agreements [Line Items]
Proceeds from royalties (percent)	24.00%
Minimum | Incyte Corporation | Incyte MGA012 Agreement
Collaboration And Other Agreements [Line Items]
Proceeds from royalties (percent)	15.00%
Revenues From License Agreements | Incyte Corporation | Incyte MGA012 Agreement
Collaboration And Other Agreements [Line Items]
Revenues	$ 150,000
Revenues From License Agreements | Incyte Corporation | Incyte MGA012 Agreement - Clinical activities
Collaboration And Other Agreements [Line Items]
Revenues		100	2,500
Revenues From License Agreements | Incyte Corporation | Incyte MGA012 Agreement - Services
Collaboration And Other Agreements [Line Items]
Revenues		$ 4,000	$ 0