Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 29, 2012	Mar. 31, 2018	Mar. 31, 2017
Schedule of Equity Method Investments [Line Items]
Collaboration profit (loss) sharing		$ 42.5	$ 20.8
Revenue related to technical development and technology transfer services		$ 164.4	90.0
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity		5.40%
Percentage of stake in entity maximum	49.90%
Biogen share of co-promotion profits or losses		50.00%
Collaboration profit (loss) sharing		$ 43.8	20.8
Revenue related to technical development and technology transfer services		$ 17.9	$ 2.2