Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,775.0	$ 3,681.6	$ 5,469.0	$ 7,215.9
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	578.5	1,190.3	1,131.4	2,291.1
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	269.1	921.7	505.1	1,699.2
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	309.4	268.6	626.3	591.9
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	400.4	481.4	800.9	947.4
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	257.4	345.6	499.2	638.2
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	143.0	135.8	301.7	309.2
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	524.2	432.0	1,027.5	954.4
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	299.8	244.1	573.1	521.8
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	224.4	187.9	454.4	432.6
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.1	23.0	52.7	51.3
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	26.1	23.0	52.7	51.3
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,529.2	2,126.7	3,012.5	4,244.2
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	826.3	1,511.4	1,577.4	2,859.2
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	702.9	615.3	1,435.1	1,385.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	499.7	494.6	1,020.2	1,059.6
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	149.3	210.3	298.0	445.7
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	350.4	284.3	722.2	613.9
ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	1.6	0.0	1.6	0.0
ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1.6	0.0	1.6	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	121.5	106.2	243.2	239.7
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	121.5	106.2	243.2	239.7
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	55.6	44.8	113.5	106.4
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	55.6	44.8	113.5	106.4
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	25.3	20.6	50.8	44.3
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	25.3	20.6	50.8	44.3
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	202.4	171.6	407.5	390.4
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	202.4	171.6	407.5	390.4
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	3.1	2.8	5.9	6.1
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	3.1	2.8	5.9	6.1
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,236.0	2,795.7	4,447.7	5,700.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	977.2	1,721.7	1,877.0	3,304.9
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,258.8	$ 1,074.0	$ 2,570.7	$ 2,395.4