License agreements - Novartis (Details) $ in Thousands	1 Months Ended					3 Months Ended		6 Months Ended		12 Months Ended									116 Months Ended
	Apr. 30, 2016 USD ($)	Sep. 30, 2014 USD ($)	Jan. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Nov. 30, 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, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Jun. 30, 2019 USD ($)
License agreements
Revenues						$ 529,932	$ 521,516	$ 1,027,789	$ 903,798
JAKAFI
License agreements
Revenues						409,506	345,624	785,117	659,344
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						26,800		26,800		$ 18,600									$ 26,800
Number of deliverables under license agreement | item					2
Reimbursable costs recorded as deferred revenue				$ 10,900
Reimbursable costs included in accounts receivable						300		300		700									300
Research and development expenses reimbursed						400	700	1,000	1,200
Product royalties in accounts receivable						56,900		56,900		55,400									56,900
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						20,100	16,900	33,500	27,300
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						$ 56,900	$ 47,100	$ 102,500	$ 88,400
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