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License agreements - Novartis (Details) $ in Millions	1 Months Ended					3 Months Ended		12 Months Ended								101 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 ($) item	Mar. 31, 2018 USD ($)	Mar. 31, 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 ($)	Mar. 31, 2018 USD ($)
License agreements
Revenues						$ 382.3
JAKAFI
License agreements
Revenues						313.7
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
Number of deliverables under license agreement \| item					2
Reimbursable costs recorded as deferred revenue				$ 10.9
Reimbursable costs included in accounts receivable						1.4		$ 1.6								$ 1.4
Research and development expenses reimbursed						0.5	$ 0.7
Product royalties in accounts receivable						41.3		47.7								41.3
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						10.4	7.8
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						$ 41.3	28.8
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	$ 25.0