Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	$ (128.6)	$ (27.3)	$ (186.1)	$ 132.6
Income Tax Expense (Benefit)	240.8	211.3	979.0	881.9
Product revenues	(3,376.1)	(3,600.1)	(10,592.0)	(10,706.6)
Net income attributable to Biogen Inc.	701.5	1,545.9	3,642.7	4,448.8
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income attributable to Biogen Inc.	(6.6)	37.1	35.9	85.6
Unrealized gains (losses) on securities available for sale | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	0.1	0.6	(12.4)	1.6
Income Tax Expense (Benefit)	0.0	(0.1)	2.6	(0.3)
Unrealized gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	0.0	0.1	0.2	0.2
Income Tax Expense (Benefit)	0.1	(0.1)	(0.1)	(0.3)
Product revenues	9.1	35.2	(41.6)	79.8
Operating Expenses	1.5	(0.8)	1.4	2.0
Gains (losses) on net investment hedge | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	$ 0.8	$ 2.2	$ 2.6	$ 6.6