Collaborative and Other Relationships Collaborative and Other Relationships (Details 3) - Eisai - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Collaborative arrangements and non-collaborative arrangement transactions
Total expense incurred by collaboration	$ 84.1	$ 57.5	$ 0.0
Biogen's share of expense reflected within our consolidation statements of income	$ 40.4	$ 29.1	$ 0.0

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