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Revenues - Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Fumarate
Disaggregation of Revenue [Line Items]
Revenues									3,905.4	4,438.2	4,274.1
Interferon
Disaggregation of Revenue [Line Items]
Revenues									1,877.5	2,101.8	2,363.0
TYSABRI
Disaggregation of Revenue [Line Items]
Revenues									1,946.1	1,892.2	1,864.0
FAMPYRA
Disaggregation of Revenue [Line Items]
Revenues									103.1	97.1	92.7
ZINBRYTA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	1.4
Subtotal: MS
Disaggregation of Revenue [Line Items]
Revenues									7,832.1	8,529.3	8,595.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Revenues									2,052.1	2,097.0	1,724.2
BENEPALI
Disaggregation of Revenue [Line Items]
Revenues									481.6	486.2	485.2
IMRALDI
Disaggregation of Revenue [Line Items]
Revenues									216.3	184.0	16.7
FLIXABI
Disaggregation of Revenue [Line Items]
Revenues									97.9	68.1	43.2
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Revenues									795.8	738.3	545.1
FUMADERM
Disaggregation of Revenue [Line Items]
Revenues									12.2	15.2	22.3
Total product revenues
Disaggregation of Revenue [Line Items]
Revenues	$ 2,301.6	$ 2,690.3	$ 2,795.7	$ 2,904.6	$ 2,924.8	$ 2,894.7	$ 2,880.3	$ 2,680.0	10,692.2	11,379.8	10,886.8
United States | Fumarate
Disaggregation of Revenue [Line Items]
Revenues									2,742.0	3,312.0	3,253.2
United States | Interferon
Disaggregation of Revenue [Line Items]
Revenues									1,273.5	1,426.6	1,668.3
United States | TYSABRI
Disaggregation of Revenue [Line Items]
Revenues									1,096.8	1,041.8	1,025.0
United States | FAMPYRA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | ZINBRYTA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | Subtotal: MS
Disaggregation of Revenue [Line Items]
Revenues									5,112.3	5,780.4	5,946.5
United States | SPINRAZA
Disaggregation of Revenue [Line Items]
Revenues									787.8	933.4	854.0
United States | BENEPALI
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | IMRALDI
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | FLIXABI
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | FUMADERM
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States | Total product revenues
Disaggregation of Revenue [Line Items]
Revenues									5,900.1	6,713.8	6,800.5
Rest of World | Fumarate
Disaggregation of Revenue [Line Items]
Revenues									1,163.4	1,126.2	1,020.9
Rest of World | Interferon
Disaggregation of Revenue [Line Items]
Revenues									604.0	675.2	694.7
Rest of World | TYSABRI
Disaggregation of Revenue [Line Items]
Revenues									849.3	850.4	839.0
Rest of World | FAMPYRA
Disaggregation of Revenue [Line Items]
Revenues									103.1	97.1	92.7
Rest of World | ZINBRYTA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	1.4
Rest of World | Subtotal: MS
Disaggregation of Revenue [Line Items]
Revenues									2,719.8	2,748.9	2,648.7
Rest of World | SPINRAZA
Disaggregation of Revenue [Line Items]
Revenues									1,264.3	1,163.6	870.2
Rest of World | BENEPALI
Disaggregation of Revenue [Line Items]
Revenues									481.6	486.2	485.2
Rest of World | IMRALDI
Disaggregation of Revenue [Line Items]
Revenues									216.3	184.0	16.7
Rest of World | FLIXABI
Disaggregation of Revenue [Line Items]
Revenues									97.9	68.1	43.2
Rest of World | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Revenues									795.8	738.3	545.1
Rest of World | FUMADERM
Disaggregation of Revenue [Line Items]
Revenues									12.2	15.2	22.3
Rest of World | Total product revenues
Disaggregation of Revenue [Line Items]
Revenues									$ 4,792.1	$ 4,666.0	$ 4,086.3

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