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Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,778.9	$ 3,376.1	$ 8,247.9	$ 10,592.0
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	619.5	967.5	1,750.9	3,258.6
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	299.9	684.2	805.1	2,383.4
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	319.6	283.3	945.8	875.2
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	387.5	474.1	1,188.4	1,421.5
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	252.4	327.3	751.6	965.5
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	135.1	146.8	436.8	456.0
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	522.8	516.5	1,550.4	1,470.9
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	281.1	304.2	854.2	826.0
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	241.7	212.3	696.2	644.9
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.2	26.8	78.8	78.1
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	26.2	26.8	78.8	78.1
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,556.0	1,984.9	4,568.5	6,229.1
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	833.4	1,315.7	2,410.9	4,174.9
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	722.6	669.2	2,157.6	2,054.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	444.1	494.4	1,464.4	1,554.0
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	139.8	182.5	437.8	628.2
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	304.3	311.9	1,026.6	925.8
ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	0.3	0.0	2.0	0.0
ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.3	0.0	2.0	0.0
ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	120.8	124.2	363.9	363.9
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	120.8	124.2	363.9	363.9
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.4	56.2	170.9	162.6
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	57.4	56.2	170.9	162.6
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	24.6	27.5	75.4	71.8
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	24.6	27.5	75.4	71.8
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	202.8	207.9	610.2	598.3
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	202.8	207.9	610.2	598.3
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.5	3.1	8.3	9.2
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	2.5	3.1	8.3	9.2
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,205.7	2,690.3	6,653.4	8,390.6
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	973.5	1,498.2	2,850.7	4,803.1
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,232.2	$ 1,192.1	$ 3,802.7	$ 3,587.5

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