Revenue - Revenue From Licensing Agreements and Strategic Partnerships (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue	$ 5,332	$ 11,691	$ 13,877	$ 15,843
Seattle Genetics
Disaggregation of Revenue [Line Items]
Total Revenue	504	2,425	1,429	2,731
AstraZeneca
Disaggregation of Revenue [Line Items]
Total Revenue	3,888	6,390	9,908	8,937
Servier
Disaggregation of Revenue [Line Items]
Total Revenue	940	1,419	2,540	2,627
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 0	$ 1,457	$ 0	$ 1,548