Collaborative and Other Relationships Collaborative and Other Relationships (Details 3) - Eisai - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Collaborative arrangements and non-collaborative arrangement transactions
Total expense incurred by collaboration	$ 95.1	$ 84.1	$ 57.5
Biogen's share of expense reflected within our consolidation statements of income	$ 50.5	$ 40.4	$ 29.1

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