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Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	1 Months Ended		3 Months Ended
	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 KRW (₩)	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 KRW (₩)	Nov. 07, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Loss recorded on Samsung Bioepis joint venture			$ 0.0	$ 0.0
Our share of Samsung Bioepis gains (losses)			14.8	28.7
Collaboration profit (loss) sharing			71.8	58.1
Total revenues			$ 3,534.3	$ 3,489.8
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			$ 676.6
Amortization of basis differences										$ 675.0
Loss recorded on Samsung Bioepis joint venture			$ 14.8
Our share of Samsung Bioepis gains (losses)			(6.1)
Amortization of basis differences			20.9
Investment in Samsung Bioepis			$ 531.6			₩ 651.1		$ 580.2	₩ 670.8
Biogen share of co-promotion profits or losses			50.00%
Collaboration profit (loss) sharing			$ 71.8	$ 58.1
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Upfront and milestone payments made to collaborative partner			100.0
Research and development expense			63.0
Prepaid research and development expenses							$ 37.0
Estimated additional payments upon achievement of development and commercial milestones			210.0
Additional Milestone Payment							$ 15.0
Contract Option Exercise Fee			60.0
Due from Related Parties			3.8					85.0
Due to Related Parties			19.8					$ 100.0
Collaborative arrangement \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Total revenues			$ 3.7	$ 24.8
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