Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Product revenue, net
Disaggregation of Revenue [Line Items]
Total revenue	1,752.3	1,726.5
Product revenue, net | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	839.2	753.8
Product revenue, net | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	913.1	972.7
MS Product Revenues
Disaggregation of Revenue [Line Items]
Total revenue	957.5	953.0
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	559.4	490.4
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	398.1	462.6
Fumarate
Disaggregation of Revenue [Line Items]
Total revenue	288.5	344.9
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	184.8	156.9
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	103.7	188.0
TECFIDERA
Disaggregation of Revenue [Line Items]
Total revenue	109.5	206.1
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	31.4	39.8
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	78.1	166.3
VUMERITY
Disaggregation of Revenue [Line Items]
Total revenue	179.0	138.8
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	153.4	117.1
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	25.6	21.7
Interferon
Disaggregation of Revenue [Line Items]
Total revenue	227.5	226.3
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	132.8	132.7
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	94.7	93.6
AVONEX
Disaggregation of Revenue [Line Items]
Total revenue	163.2	166.8
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	108.5	108.6
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	54.7	58.2
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total revenue	64.3	59.5
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	24.3	24.1
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	40.0	35.4
TYSABRI
Disaggregation of Revenue [Line Items]
Total revenue	441.5	381.5
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	241.8	200.8
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	199.7	180.7
FAMPYRA
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.3
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	0.0	0.3
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Total revenue	557.2	563.3
Rare Disease Product Revenue | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	224.5	231.0
Rare Disease Product Revenue | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	332.7	332.3
SPINRAZA
Disaggregation of Revenue [Line Items]
Total revenue	374.0	423.9
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	142.2	154.4
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	231.8	269.5
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total revenue	150.7	123.9
SKYCLARYS | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	71.8	69.1
SKYCLARYS | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	78.9	54.8
QALSODY
Disaggregation of Revenue [Line Items]
Total revenue	32.5	15.5
QALSODY | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	10.5	7.5
QALSODY | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	22.0	8.0
Biosimilars
Disaggregation of Revenue [Line Items]
Total revenue	182.2	180.8
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	4.3
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	182.2	176.5
BENEPALI
Disaggregation of Revenue [Line Items]
Total revenue	122.1	111.3
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	122.1	111.3
IMRALDI
Disaggregation of Revenue [Line Items]
Total revenue	49.6	47.4
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	49.6	47.4
FLIXABI
Disaggregation of Revenue [Line Items]
Total revenue	10.5	13.1
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	10.5	13.1
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total revenue	0.0	8.9
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	4.2
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	4.7
TOFIDENCE
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.1
TOFIDENCE | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.1
TOFIDENCE | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Total revenue	55.4	29.4
Other | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	55.3	28.1
Other | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.1	1.3
ZURZUVAE
Disaggregation of Revenue [Line Items]
Total revenue	55.4	27.7
ZURZUVAE | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	55.3	27.7
ZURZUVAE | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.1	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total revenue	0.0	1.7
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.4
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	$ 0.0	$ 1.3