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License agreements - Novartis (Details) $ in Millions	1 Months Ended					3 Months Ended		9 Months Ended		12 Months Ended								107 Months Ended
License agreements - Novartis (Details) $ in Millions	Apr. 30, 2016 USD ($)	Sep. 30, 2014 USD ($)	Jan. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Nov. 30, 2009 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 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 ($)	Sep. 30, 2018 USD ($)
License agreements
Revenues						$ 449.7		$ 1,353.5
Milestone revenues
License agreements
Revenues						20.0		120.0
Novartis
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 210.0
Upfront payment received under license agreement				$ 150.0
Immediate milestone payment received under license agreement			$ 60.0
Royalties payable						17.0		17.0		$ 14.8								$ 17.0
Number of deliverables under license agreement \| item					2
Reimbursable costs recorded as deferred revenue				$ 10.9
Reimbursable costs included in accounts receivable						0.4		0.4		1.6								0.4
Research and development expenses reimbursed						1.2	$ 0.4	2.4	$ 1.9
Product royalties in accounts receivable						50.9		50.9		47.7								50.9
Novartis \| Pre-specified Events \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 1,200.0
Novartis \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone																		132.0
Novartis \| Development Milestones \| Phase III
License agreements
Amount recognized and received for the achievement of a predefined milestone																	$ 50.0
Novartis \| Development Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					174.0
Novartis \| Regulatory Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone																		215.0
Novartis \| Regulatory Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					495.0
Novartis \| Commercialization Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone																		$ 60.0
Novartis \| Commercialization Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 500.0
Novartis \| LY3009104 \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone												$ 5.0	$ 7.0	$ 25.0		$ 15.0
Novartis \| GVHD \| Development Milestones \| Phase III
License agreements
Amount recognized and received for the achievement of a predefined milestone										25.0
Novartis \| GVHD \| Development and Regulatory Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement	$ 75.0
Novartis \| GVHD \| Development and Commercialization Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone											$ 5.0
Novartis \| JAKAFI \| U.S.
License agreements
Royalties payable on net sales						17.0	14.9	44.3	35.7
Novartis \| JAKAFI \| Regulatory Milestones \| Europe
License agreements
Amount recognized and received for the achievement of a predefined milestone											$ 40.0	25.0	60.0		$ 40.0
Novartis \| JAKAFI \| Regulatory Milestones \| JAPAN
License agreements
Amount recognized and received for the achievement of a predefined milestone													$ 25.0
Novartis \| JAKAFI \| Regulatory Milestones \| U.S.
License agreements
Amount recognized and received for the achievement of a predefined milestone																$ 10.0
Novartis \| JAKAVI
License agreements
Revenues						50.9	41.3	$ 139.4	104.0
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		$ 60.0
Novartis \| JAKAVI \| Regulatory Milestones \| JAPAN
License agreements
Amount recognized and received for the achievement of a predefined milestone												15.0
Novartis \| JAKAVI \| Commercialization Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone										40.0		20.0
Revenues										$ 600.0		$ 300.0
Novartis \| Milestone revenues
License agreements
Revenues						$ 0.0	$ 0.0	$ 0.0	$ 25.0