Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,463.0	$ 2,531.8
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,125.2	1,394.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	546.1	709.1
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	579.1	685.4
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	382.7	537.9
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	168.2	242.3
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	214.5	295.6
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	274.5	409.9
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	74.7	117.1
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	199.8	292.8
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	108.2	128.0
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	93.5	125.2
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	14.7	2.8
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	245.6	309.6
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	132.5	182.3
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	113.1	127.3
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	172.4	229.6
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	102.6	148.0
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	69.8	81.6
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	73.2	80.0
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	29.9	34.3
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	43.3	45.7
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	472.8	520.8
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	245.4	284.5
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	227.4	236.3
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	24.1	26.2
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	24.1	26.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	443.3	472.5
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	146.7	163.3
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	296.6	309.2
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	192.4	194.3
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	8.2	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	184.2	194.3
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	109.0	114.7
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	109.0	114.7
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	54.4	57.1
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	54.4	57.1
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	20.4	22.5
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	20.4	22.5
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	8.6	0.0
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	8.2	0.0
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.4	0.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,763.3	2,066.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	701.4	875.2
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1,061.9	1,191.1
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	2.4	5.0
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.4	2.8
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 2.0	$ 2.2