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License agreements - Novartis (Details) $ in Thousands	1 Months Ended					3 Months Ended		9 Months Ended		12 Months Ended									119 Months Ended
License agreements - Novartis (Details) $ in Thousands	Apr. 30, 2016 USD ($)	Sep. 30, 2014 USD ($)	Jan. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Nov. 30, 2009 USD ($) item	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Sep. 30, 2019 USD ($)
License agreements
Revenues						$ 551,581	$ 449,683	$ 1,579,370	$ 1,353,481
JAKAFI
License agreements
Revenues						433,387	347,567	1,218,504	1,006,911
Novartis
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 210,000
Upfront payment received under license agreement				$ 150,000
Immediate milestone payment received under license agreement			$ 60,000
Revenues						0		0
Royalties payable						38,000		38,000		$ 18,600									$ 38,000
Number of deliverables under license agreement \| item					2
Reimbursable costs recorded as deferred revenue				$ 10,900
Reimbursable costs included in accounts receivable						600		600		700									600
Research and development expenses reimbursed						0	1,200	1,000	2,400
Product royalties in accounts receivable						58,400		58,400		55,400									58,400
Novartis \| Pre-specified Events \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 1,200,000
Novartis \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone																			132,000
Novartis \| Development Milestones \| Phase III
License agreements
Amount recognized and received for the achievement of a predefined milestone																		$ 50,000
Novartis \| Development Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					174,000
Novartis \| Regulatory Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone																			215,000
Novartis \| Regulatory Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					495,000
Novartis \| Commercialization Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone										60,000									$ 120,000
Revenues										$ 900,000
Novartis \| Commercialization Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 500,000
Novartis \| LY3009104 \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone													$ 5,000	$ 7,000	$ 25,000		$ 15,000
Novartis \| GVHD \| Development Milestones \| Phase III
License agreements
Amount recognized and received for the achievement of a predefined milestone											$ 25,000
Novartis \| GVHD \| Development and Regulatory Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement	$ 75,000
Novartis \| GVHD \| Development and Commercialization Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone												$ 5,000
Novartis \| JAKAFI \| U.S.
License agreements
Royalties payable on net sales						21,200	17,000	54,700	44,300
Novartis \| JAKAFI \| Regulatory Milestones \| Europe
License agreements
Amount recognized and received for the achievement of a predefined milestone												$ 40,000	25,000	60,000		$ 40,000
Novartis \| JAKAFI \| Regulatory Milestones \| JAPAN
License agreements
Amount recognized and received for the achievement of a predefined milestone														$ 25,000
Novartis \| JAKAFI \| Regulatory Milestones \| U.S.
License agreements
Amount recognized and received for the achievement of a predefined milestone																	$ 10,000
Novartis \| JAKAVI
License agreements
Revenues						$ 58,400	$ 50,900	$ 160,900	$ 139,400
Novartis \| JAKAVI \| Minimum
License agreements
Royalty payments on future global net sales (as a percent)								12.00%
Novartis \| JAKAVI \| Maximum
License agreements
Royalty payments on future global net sales (as a percent)								14.00%
Novartis \| JAKAVI \| Regulatory Milestones \| Europe
License agreements
Amount recognized and received for the achievement of a predefined milestone		$ 60,000
Novartis \| JAKAVI \| Regulatory Milestones \| JAPAN
License agreements
Amount recognized and received for the achievement of a predefined milestone													15,000
Novartis \| JAKAVI \| Commercialization Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone											40,000		20,000
Revenues											$ 600,000		$ 300,000