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Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	3 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 KRW (₩)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 KRW (₩)	Dec. 31, 2018 KRW (₩)	Nov. 07, 2018 USD ($)	Jun. 30, 2018
Schedule of Equity Method Investments [Line Items]
Our share of Samsung Bioepis gains (losses)	$ (28.7)			$ 0.0
Collaboration profit (loss) sharing	58.1			42.5
Total revenues	$ 3,489.8			3,131.1
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity								5.00%
Percentage of stake in entity maximum	49.90%				49.90%
Payments to acquire additional investment in equity method investment		$ 676.6	₩ 759.5
Amortization of basis differences							$ 675.0
Loss recorded on Samsung Bioepis joint venture	$ 28.7
Our share of Samsung Bioepis gains (losses)	14.0
Amortization of basis differences	14.7
Investment in Samsung Bioepis	$ 642.1	$ 680.6			₩ 729.9	₩ 759.5
Biogen share of co-promotion profits or losses	50.00%
Collaboration profit (loss) sharing	$ 58.1			43.8
Collaborative arrangement | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Total revenues	$ 24.8			$ 17.9
Inventory | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							115.0
Equity method investment basis difference amortization period	1 year 6 months
Developed technology | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							615.0
Equity method investment basis difference amortization period	15 years
In Process Research and Development | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							170.0
Deferred tax liability | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							$ 225.0

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