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License agreements - Novartis (Details) $ in Thousands	1 Months Ended					3 Months Ended		9 Months Ended		12 Months Ended							95 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 ($) item	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 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, 2017 USD ($)
License agreements
Contract revenues						$ 15,000	$ 3,214	$ 105,000	$ 69,643
Product royalty revenues						44,487	29,626	108,477	77,486
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
Number of deliverables under license agreement \| item					2
Reimbursable costs recorded as deferred revenue				$ 10,900
Reimbursable costs included in accounts receivable						1,000		1,000		$ 600							$ 1,000
Research and development expenses reimbursed						400	300	1,900	700
Contract revenues						0	0	25,000	5,000
Product royalties in accounts receivable						41,300		41,300		33,300							41,300
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 \| 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 \| Europe
License agreements
Amount recognized and received for the achievement of a predefined milestone										40,000
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																	$ 20,000
Novartis \| Commercialization Milestones \| Maximum
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 500,000
Novartis \| GVHD
License agreements
Additional milestone payment received under license agreement	$ 75,000
Novartis \| GVHD \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone								25,000
Novartis \| GVHD \| Commercialization Milestones \| Non-U.S.
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						14,900	11,000	35,700	25,100
Novartis \| JAKAFI \| Development Milestones
License agreements
Amount recognized and received for the achievement of a predefined milestone											$ 5,000	$ 7,000	$ 25,000		$ 15,000	$ 50,000
Novartis \| JAKAFI \| Regulatory Milestones \| Europe
License agreements
Amount recognized and received for the achievement of a predefined milestone											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 \| Capmatinib \| Minimum
License agreements
Royalty payments on future global net sales (as a percent)					12.00%
Novartis \| Capmatinib \| Maximum
License agreements
Royalty payments on future global net sales (as a percent)					14.00%
Novartis \| JAKAVI
License agreements
Net sales											300,000
Product royalty revenues						$ 41,300	$ 29,600	$ 104,000	$ 77,500
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											$ 20,000