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Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	1 Months Ended		3 Months Ended					6 Months Ended
	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 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]
Our share of Samsung Bioepis gains (losses)			$ (15.1)		$ 16.3			$ (0.4)	$ 45.0
Collaboration profit (loss) sharing			21.8		63.5			93.5	121.6
Total revenues			$ 3,681.6		$ 3,616.7			$ 7,215.9	$ 7,106.5
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity					5.00%				5.00%
Percentage of stake in entity maximum			49.90%					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			$ 15.1					$ 0.4
Investment in Samsung Bioepis			557.5					$ 557.5		₩ 668.3		$ 580.2	₩ 670.8
Biogen share of co-promotion profits or losses								50.00%
Collaboration profit (loss) sharing			55.4		$ 63.4			$ 127.2	$ 121.5
Eisai
Schedule of Equity Method Investments [Line Items]
Loss on research and development contracts terminated with Eisai						$ 45.0
Our share of Samsung Bioepis gains (losses)			(33.8)					(33.8)
Additional Milestone Payment			75.0					75.0
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			13.3					13.3				85.0
Due to Related Parties			0.0					0.0				100.0
Collaborative arrangement \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Total revenues			$ 4.5		$ 52.2			$ 8.2	$ 77.0
Inventory \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period								1 year 6 months
Developed technology \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period								15 years
Neurimmune \| Regulatory Milestones [Member]
Schedule of Equity Method Investments [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately												$ 75.0