Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,694.0	$ 3,534.3
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	552.9	1,100.8
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	236.0	777.5
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	316.9	323.3
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	400.5	466.0
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	241.8	292.6
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	158.7	173.4
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	503.3	522.4
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	273.3	277.7
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	230.0	244.7
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.6	28.3
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	26.6	28.3
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,483.3	2,117.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	751.1	1,347.8
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	732.2	769.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	520.5	565.0
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	148.7	235.4
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	371.8	329.6
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	121.7	133.5
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	121.7	133.5
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.9	61.6
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	57.9	61.6
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	25.5	23.7
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	25.5	23.7
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	205.1	218.8
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	205.1	218.8
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.8	3.3
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	2.8	3.3
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,211.7	2,904.6
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	899.8	1,583.2
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,311.9	$ 1,321.4