Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,290.5	$ 2,463.0
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,075.9	1,125.2
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	503.2	546.1
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	572.7	579.1
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	381.8	382.7
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	149.6	168.2
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	232.2	214.5
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	254.3	274.5
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	43.7	74.7
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	210.6	199.8
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	127.5	108.2
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	105.9	93.5
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	21.6	14.7
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	243.6	245.6
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	139.8	132.5
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	103.8	113.1
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	178.5	172.4
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	111.2	102.6
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	67.3	69.8
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	65.1	73.2
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	28.6	29.9
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	36.5	43.3
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	431.3	472.8
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	213.8	245.4
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	217.5	227.4
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	19.2	24.1
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	19.2	24.1
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Product revenues	423.9	443.3
Rare Disease Product Revenue | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	225.9	146.7
Rare Disease Product Revenue | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	198.0	296.6
QALSODY
Disaggregation of Revenue [Line Items]
Product revenues	4.6	0.0
QALSODY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	4.4	0.0
QALSODY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.2	0.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	341.3	443.3
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	148.5	146.7
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	192.8	296.6
Completed technology for SKYCLARYS (U.S.)
Disaggregation of Revenue [Line Items]
Product revenues	78.0	0.0
Completed technology for SKYCLARYS (U.S.) | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	73.0	0.0
Completed technology for SKYCLARYS (U.S.) | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	5.0	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	196.9	192.4
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	3.7	8.2
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	193.2	184.2
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	118.7	109.0
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	118.7	109.0
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	54.8	54.4
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	54.8	54.4
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	17.8	20.4
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	17.8	20.4
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	5.6	8.6
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	3.7	8.2
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1.9	0.4
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,711.9	1,763.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	746.1	701.4
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	965.8	1,061.9
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	15.2	2.4
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	13.3	0.4
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1.9	$ 2.0