Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,456.0	$ 2,589.1	$ 4,919.0	$ 5,120.9
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,209.3	1,426.6	2,334.5	2,821.1
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	636.7	753.7	1,182.8	1,462.8
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	572.6	672.9	1,151.7	1,358.3
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	400.4	534.7	783.1	1,072.6
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	196.8	250.6	365.0	492.9
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	203.6	284.1	418.1	579.7
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	254.2	397.9	528.7	807.8
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	66.5	120.7	141.2	237.8
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	187.7	277.2	387.5	570.0
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	146.2	136.8	254.4	264.8
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	130.3	129.9	223.8	255.1
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	15.9	6.9	30.6	9.7
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	302.4	350.2	548.0	659.8
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	180.0	211.2	312.5	393.5
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	122.4	139.0	235.5	266.3
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	220.3	258.7	392.7	488.3
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	145.9	171.0	248.5	319.0
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	74.4	87.7	144.2	169.3
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	82.1	91.5	155.3	171.5
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	34.1	40.2	64.0	74.5
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	48.0	51.3	91.3	97.0
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	483.1	516.2	955.9	1,037.0
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	259.9	291.9	505.3	576.4
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	223.2	224.3	450.6	460.6
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	23.4	25.5	47.5	51.7
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	23.4	25.5	47.5	51.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	437.1	431.1	880.4	903.6
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	155.8	139.8	302.5	303.1
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	281.3	291.3	577.9	600.5
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	195.1	194.4	387.5	388.7
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	7.0	0.5	15.2	0.5
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	188.1	193.9	372.3	388.2
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	109.2	115.8	218.2	230.5
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	109.2	115.8	218.2	230.5
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	58.8	57.6	113.2	114.7
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	58.8	57.6	113.2	114.7
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	20.1	20.5	40.5	43.0
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	20.1	20.5	40.5	43.0
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	7.0	0.5	15.6	0.5
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	7.0	0.5	15.2	0.5
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.4	0.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,845.8	2,054.9	3,609.1	4,121.2
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	801.0	894.1	1,502.4	1,769.3
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1,044.8	1,160.8	2,106.7	2,351.9
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	4.3	2.8	6.7	7.8
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1.5	0.1	1.9	2.9
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	2.8	2.7	4.8	4.9
Spinal Muscular Atrophy
Disaggregation of Revenue [Line Items]
Product revenues	437.1	431.1	880.4	903.6
Spinal Muscular Atrophy | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	155.8	139.8	302.5	303.1
Spinal Muscular Atrophy | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 281.3	$ 291.3	$ 577.9	$ 600.5