Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,465.8	$ 2,530.3	$ 7,221.2	$ 7,449.3
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,769.4	1,805.2	5,380.9	5,414.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	813.0	790.9	2,405.8	2,293.3
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	956.4	1,014.3	2,975.1	3,121.0
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,053.9	1,159.0	3,279.6	3,493.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	546.0	631.8	1,631.6	1,814.6
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	507.9	527.2	1,648.0	1,678.9
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	390.9	405.0	1,190.7	1,188.1
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	175.0	206.9	512.9	571.9
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	215.9	198.1	677.8	616.2
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	232.8	239.5	739.3	768.2
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	40.1	58.1	127.9	199.3
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	192.7	181.4	611.4	568.9
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	158.1	165.5	451.4	419.9
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	134.9	148.8	385.0	372.6
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	23.2	16.7	66.4	47.3
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	237.5	277.7	732.0	825.7
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	143.5	180.1	428.7	492.6
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	94.0	97.6	303.3	333.1
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	176.2	212.2	537.5	604.9
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	115.6	148.7	344.0	397.2
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	60.6	63.5	193.5	207.7
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	61.3	65.5	194.5	220.8
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	27.9	31.4	84.7	95.4
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	33.4	34.1	109.8	125.4
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	406.1	456.3	1,299.6	1,412.2
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	227.5	244.8	690.0	750.1
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	178.6	211.5	609.6	662.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	19.4	20.0	57.3	67.5
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	19.4	20.0	57.3	67.5
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Product revenues	494.8	449.9	1,452.8	1,331.2
Rare Disease Product Revenue | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	240.4	152.1	703.8	455.5
Rare Disease Product Revenue | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	254.4	297.8	749.0	875.7
QALSODY
Disaggregation of Revenue [Line Items]
Product revenues	11.1	1.7	20.7	2.6
QALSODY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	5.5	1.6	14.5	2.5
QALSODY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	5.6	0.1	6.2	0.1
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	381.4	448.2	1,151.8	1,328.6
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	153.1	150.5	458.9	453.0
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	228.3	297.7	692.9	875.6
Completed technology for SKYCLARYS (U.S.)
Disaggregation of Revenue [Line Items]
Product revenues	102.3	0.0	280.3	0.0
Completed technology for SKYCLARYS (U.S.) | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	81.8	0.0	230.4	0.0
Completed technology for SKYCLARYS (U.S.) | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	20.5	0.0	49.9	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	196.6	194.3	591.6	581.8
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	4.3	6.1	19.1	21.3
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	192.3	188.2	572.5	560.5
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	118.1	112.8	354.1	331.0
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	118.1	112.8	354.1	331.0
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	54.1	54.4	162.1	167.6
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	54.1	54.4	162.1	167.6
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	16.2	20.2	47.1	60.7
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	16.2	20.2	47.1	60.7
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	8.0	6.9	27.3	22.5
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	4.1	6.1	18.1	21.3
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	3.9	0.8	9.2	1.2
TOFIDENCE
Disaggregation of Revenue [Line Items]
Product revenues	0.2	0.0	1.0	0.0
TOFIDENCE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.2	0.0	1.0	0.0
TOFIDENCE | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Product revenues	24.1	2.0	56.9	7.8
Other | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	22.3	0.9	51.3	1.9
Other | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1.8	1.1	5.6	5.9
ZURZUVAE
Disaggregation of Revenue [Line Items]
Product revenues	22.0	0.0	49.3	0.0
ZURZUVAE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	22.0	0.0	49.3	0.0
ZURZUVAE | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	2.1	2.0	7.6	7.8
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.3	0.9	2.0	1.9
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1.8	$ 1.1	$ 5.6	$ 5.9