Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,431.0	$ 2,290.5
Product revenue, net
Disaggregation of Revenue [Line Items]
Total revenue	1,726.5	1,711.9
Product revenue, net | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	753.8	746.1
Product revenue, net | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	972.7	965.8
MS Product Revenues
Disaggregation of Revenue [Line Items]
Total revenue	953.0	1,075.9
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	490.4	503.2
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	462.6	572.7
Fumarate
Disaggregation of Revenue [Line Items]
Total revenue	344.9	381.8
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	156.9	149.6
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	188.0	232.2
TECFIDERA
Disaggregation of Revenue [Line Items]
Total revenue	206.1	254.3
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	39.8	43.7
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	166.3	210.6
VUMERITY
Disaggregation of Revenue [Line Items]
Total revenue	138.8	127.5
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	117.1	105.9
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	21.7	21.6
Interferon
Disaggregation of Revenue [Line Items]
Total revenue	226.3	243.6
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	132.7	139.8
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	93.6	103.8
AVONEX
Disaggregation of Revenue [Line Items]
Total revenue	166.8	178.5
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	108.6	111.2
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	58.2	67.3
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total revenue	59.5	65.1
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	24.1	28.6
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	35.4	36.5
TYSABRI
Disaggregation of Revenue [Line Items]
Total revenue	381.5	431.3
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	200.8	213.8
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	180.7	217.5
FAMPYRA
Disaggregation of Revenue [Line Items]
Total revenue	0.3	19.2
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.3	19.2
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Total revenue	563.3	423.9
Rare Disease Product Revenue | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	231.0	225.9
Rare Disease Product Revenue | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	332.3	198.0
QALSODY
Disaggregation of Revenue [Line Items]
Total revenue	15.5	4.6
QALSODY | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	7.5	4.4
QALSODY | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	8.0	0.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Total revenue	423.9	341.3
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	154.4	148.5
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	269.5	192.8
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total revenue	123.9	78.0
SKYCLARYS | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	69.1	73.0
SKYCLARYS | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	54.8	5.0
Biosimilars
Disaggregation of Revenue [Line Items]
Total revenue	180.8	196.9
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	4.3	3.7
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	176.5	193.2
BENEPALI
Disaggregation of Revenue [Line Items]
Total revenue	111.3	118.7
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	111.3	118.7
IMRALDI
Disaggregation of Revenue [Line Items]
Total revenue	47.4	54.8
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	47.4	54.8
FLIXABI
Disaggregation of Revenue [Line Items]
Total revenue	13.1	17.8
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	13.1	17.8
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total revenue	8.9	5.6
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	4.2	3.7
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	4.7	1.9
TOFIDENCE
Disaggregation of Revenue [Line Items]
Total revenue	0.1	0.0
TOFIDENCE | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.1	0.0
TOFIDENCE | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Total revenue	29.4	15.2
Other | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	28.1	13.3
Other | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	1.3	1.9
ZURZUVAE
Disaggregation of Revenue [Line Items]
Total revenue	27.7	12.4
ZURZUVAE | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	27.7	12.4
ZURZUVAE | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total revenue	1.7	2.8
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.4	0.9
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	$ 1.3	$ 1.9