Collaborative and Other Relationships Collaborative and Other Relationships (Details 2) - Eisai - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative arrangements and non-collaborative arrangement transactions
Total expense incurred by collaboration	$ 146.2	$ 95.1	$ 84.1
Biogen's share of expense reflected within our consolidation statements of income	$ 74.3	$ 50.5	$ 40.4

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