Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,531.8	$ 2,694.0
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	537.9	552.9
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	242.3	236.0
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	295.6	316.9
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	309.6	400.5
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	182.3	241.8
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	127.3	158.7
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	520.8	503.3
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	284.5	273.3
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	236.3	230.0
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.2	26.6
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	26.2	26.6
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,394.5	1,483.3
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	709.1	751.1
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	685.4	732.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	472.5	520.5
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	163.3	148.7
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	309.2	371.8
ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	2.8	0.0
ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	2.8	0.0
ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	114.7	121.7
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	114.7	121.7
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.1	57.9
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	57.1	57.9
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	22.5	25.5
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	22.5	25.5
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	194.3	205.1
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	194.3	205.1
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.2	2.8
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
FUMADERM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	2.2	2.8
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,066.3	2,211.7
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	875.2	899.8
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,191.1	$ 1,311.9