Collaboration and Other Agreements - Incyte (Details) $ in Thousands	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) performance_obligation	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Collaboration And Other Agreements [Line Items]
Revenues		$ 18,741	$ 20,798		$ 38,996	$ 44,327
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
Incyte Corporation \| Incyte MGA012 Agreement - Clinical activities
Collaboration And Other Agreements [Line Items]
Development and regulatory milestones recognized			10,000			10,000
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				13,100
Revenues From License Agreements \| Incyte Corporation \| Incyte MGA012 Agreement - Clinical activities
Collaboration And Other Agreements [Line Items]
Revenues		0	10,500		$ 100	13,600
Revenues From License Agreements \| Incyte Corporation \| Incyte MGA012 Agreement - Services
Collaboration And Other Agreements [Line Items]
Revenues		$ 4,900	$ 6,100			$ 16,000