Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,589.1	$ 2,775.0	$ 5,120.9	$ 5,469.0
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,426.6	1,529.2	2,821.1	3,012.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	753.7	826.3	1,462.8	1,577.4
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	672.9	702.9	1,358.3	1,435.1
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	534.7	578.5	1,072.6	1,131.4
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	250.6	269.1	492.9	505.1
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	284.1	309.4	579.7	626.3
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	397.9	487.6	807.8	966.8
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	120.7	178.4	237.8	340.8
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	277.2	309.2	570.0	626.0
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	136.8	90.9	264.8	164.6
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	129.9	90.7	255.1	164.3
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	6.9	0.2	9.7	0.3
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	350.2	400.4	659.8	800.9
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	211.2	257.4	393.5	499.2
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	139.0	143.0	266.3	301.7
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	258.7	310.9	488.3	622.0
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	171.0	214.0	319.0	423.2
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	87.7	96.9	169.3	198.8
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	91.5	89.5	171.5	178.9
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	40.2	43.4	74.5	76.0
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	51.3	46.1	97.0	102.9
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	516.2	524.2	1,037.0	1,027.5
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	291.9	299.8	576.4	573.1
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	224.3	224.4	460.6	454.4
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	25.5	26.1	51.7	52.7
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	25.5	26.1	51.7	52.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	431.1	499.7	903.6	1,020.2
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	139.8	149.3	303.1	298.0
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	291.3	350.4	600.5	722.2
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	194.4	202.4	388.7	407.5
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.5	0.0	0.5	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	193.9	202.4	388.2	407.5
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	115.8	121.5	230.5	243.2
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	115.8	121.5	230.5	243.2
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.6	55.6	114.7	113.5
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	57.6	55.6	114.7	113.5
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	20.5	25.3	43.0	50.8
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	20.5	25.3	43.0	50.8
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	0.5	0.0	0.5	0.0
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.5	0.0	0.5	0.0
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.7	3.1	4.9	5.9
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	2.7	3.1	4.9	5.9
ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	0.1	1.6	2.9	1.6
ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.1	1.6	2.9	1.6
ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,054.9	2,236.0	4,121.2	4,447.7
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	894.1	977.2	1,769.3	1,877.0
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,160.8	$ 1,258.8	$ 2,351.9	$ 2,570.7