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License agreements - Lilly (Details) $ in Thousands	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended					115 Months Ended
License agreements - Lilly (Details) $ in Thousands	May 31, 2019 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2009 USD ($) item	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2010 USD ($)	Jun. 30, 2019 USD ($)
License agreements
Research and Development Expense				$ 289,363	$ 298,089	$ 559,908	$ 601,192
Revenues				529,932	521,516	1,027,789	903,798
Product royalty revenues
License agreements
Revenues				76,035	$ 55,953	137,643	$ 103,669
Eli Lilly
License agreements
Upfront payment received under license agreement			$ 90,000
Research and Development Expense				0		0
Number of deliverables under license agreement \| item			2
Product royalties in accounts receivable				$ 19,500		$ 19,500		$ 14,000					$ 19,500
Eli Lilly \| Phase IIB
License agreements
Funding of future development costs (as a percent)				30.00%	30.00%	30.00%	30.00%
Research and Development Expense					$ 14,200		$ 26,700
Accrued and other liabilities				$ 0		$ 0		23,100					0
Eli Lilly \| Pre-specified Events \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement			$ 665,000
Eli Lilly \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone								20,000	$ 30,000				149,000
Eli Lilly \| Development Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement			150,000
Eli Lilly \| Development Milestones \| Phase III
License agreements
Amount recognized and received for the achievement of a predefined milestone											$ 50,000
Eli Lilly \| Development Milestones \| Phase IIA
License agreements
Amount recognized and received for the achievement of a predefined milestone												$ 30,000
Eli Lilly \| Development Milestones \| Phase IIB
License agreements
Amount recognized and received for the achievement of a predefined milestone												$ 19,000
Eli Lilly \| Regulatory Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone								$ 100,000					$ 235,000
Eli Lilly \| Regulatory Milestones \| JAPAN
License agreements
Amount recognized and received for the achievement of a predefined milestone									15,000
Eli Lilly \| Regulatory Milestones \| U.S.
License agreements
Amount recognized and received for the achievement of a predefined milestone										$ 35,000
Eli Lilly \| Regulatory Milestones \| Europe
License agreements
Amount recognized and received for the achievement of a predefined milestone									$ 65,000	$ 20,000
Eli Lilly \| Regulatory Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement			365,000
Eli Lilly \| Commercialization Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement			$ 150,000
Eli Lilly \| GVHD
License agreements
Upfront payment under license agreement		$ 35,000
Additional milestone payments under the license agreement		$ 40,000
Eli Lilly \| GVHD \| Regulatory Milestones
License agreements
Milestone payment made under license agreement	$ 20,000
Eli Lilly \| Product royalty revenues
License agreements
Revenues				19,100	8,900	35,200	15,300
Eli Lilly \| Milestone revenues
License agreements
Revenues				$ 0	$ 100,000	$ 0	$ 100,000