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Collaboration and Other Agreements, Janssen Biotech, Inc. (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended								12 Months Ended
	Jul. 31, 2016	Jan. 31, 2015	Dec. 31, 2014	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaboration and Other Agreements [Line Items]
Revenues				$ 15,794	$ 20,798	$ 18,834	$ 4,695	$ 152,359	$ 1,663	$ 1,666	$ 2,055	$ 60,121	$ 157,742	$ 91,880
Janssen MGD011 Agreement \| Janssen Biotech Inc.
Collaboration and Other Agreements [Line Items]
Sale of common stock (in shares)			1,923,077
Proceeds of stock sale		$ 75,000	$ 75,000
Non-refundable upfront payment														50,000
Janssen MGD015 Agreement \| Janssen Biotech Inc.
Collaboration and Other Agreements [Line Items]
Non-refundable upfront payment	$ 75,000
Revenues From License Agreements \| Janssen MGD011 Agreement \| Janssen Biotech Inc.
Collaboration and Other Agreements [Line Items]
Revenues												0		2,000
Revenues From License Agreements \| Janssen MGD015 Agreement \| Janssen Biotech Inc.
Collaboration and Other Agreements [Line Items]
Revenues												$ 0	$ 600	$ 75,800