License agreements (Details) (USD $)	3 Months Ended		1 Months Ended			3 Months Ended		12 Months Ended		1 Months Ended				12 Months Ended			1 Months Ended		3 Months Ended		12 Months Ended	1 Months Ended		12 Months Ended	1 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Jan. 31, 2010 Collaboration and License Agreement with Novartis	Dec. 31, 2009 Collaboration and License Agreement with Novartis	Nov. 30, 2009 Collaboration and License Agreement with Novartis deliverable	Mar. 31, 2013 Collaboration and License Agreement with Novartis	Mar. 31, 2012 Collaboration and License Agreement with Novartis	Dec. 31, 2010 Collaboration and License Agreement with Novartis	Dec. 31, 2012 Collaboration and License Agreement with Novartis	Nov. 30, 2009 Collaboration and License Agreement with Novartis Pre-specified events	Nov. 30, 2009 Collaboration and License Agreement with Novartis Development milestones	Nov. 30, 2009 Collaboration and License Agreement with Novartis Regulatory milestones	Nov. 30, 2009 Collaboration and License Agreement with Novartis Commercialized milestones	Dec. 31, 2011 Collaboration and License Agreement with Novartis LY3009104(INCB28060)	Dec. 31, 2012 Collaboration and License Agreement with Novartis JAKAFI (ruxolitinib)	Dec. 31, 2011 Collaboration and License Agreement with Novartis JAKAFI (ruxolitinib)	Jul. 31, 2010 License, Development and Commercialization Agreement with Lilly	Dec. 31, 2009 License, Development and Commercialization Agreement with Lilly deliverable	Mar. 31, 2013 License, Development and Commercialization Agreement with Lilly	Mar. 31, 2012 License, Development and Commercialization Agreement with Lilly	Dec. 31, 2010 License, Development and Commercialization Agreement with Lilly Maximum	Dec. 31, 2009 License, Development and Commercialization Agreement with Lilly Pre-specified events Maximum	Dec. 31, 2009 License, Development and Commercialization Agreement with Lilly Development milestones Maximum	Dec. 31, 2012 License, Development and Commercialization Agreement with Lilly Development milestones Phase III	Dec. 31, 2009 License, Development and Commercialization Agreement with Lilly Regulatory milestones Maximum	Dec. 31, 2009 License, Development and Commercialization Agreement with Lilly Commercialized milestones Maximum	Dec. 31, 2010 License, Development and Commercialization Agreement with Lilly LY3009104(INCB28060) Development milestones Phase IIa	Dec. 31, 2010 License, Development and Commercialization Agreement with Lilly LY3009104(INCB28060) Development milestones Phase IIb
License agreements
Upfront and immediate milestone payment to be received under license agreement					$ 210,000,000					$ 1,100,000,000	$ 162,000,000	$ 450,000,000	$ 500,000,000									$ 665,000,000	$ 150,000,000	$ 50,000,000	$ 365,000,000	$ 150,000,000	$ 30,000,000	$ 19,000,000
Amount recognized and received for the achievement of a predefined milestone														15,000,000		10,000,000
Additional milestone payment received under license agreement								50,000,000							40,000,000
Royalty payment obligations						0
Number of deliverables under license agreement					2													2
Upfront payment received under license agreement				150,000,000														90,000,000
Immediate milestone payment received under license agreement			60,000,000
Reimbursable costs recorded as deferred revenue				10,900,000
Reimbursable costs included in accounts receivable						900,000			2,100,000
Research and development expenses reimbursed						900,000	0
Contract revenues	$ 16,737,000	$ 16,737,000				$ 13,500,000	$ 13,500,000												$ 3,200,000	$ 3,200,000
Range of royalty payments on future global net sales, upper range (as a percent)																					20.00%
Associated future global development costs from the initiation of a Phase IIb trial, if elected to co-develop, percentage																	30.00%	30.00%