Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,534.3	$ 3,489.8
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,100.8	998.8
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	777.5	717.7
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	323.3	281.1
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	466.0	500.9
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	292.6	327.3
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	173.4	173.6
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	522.4	460.4
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	277.7	245.0
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	244.7	215.4
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	28.3	22.9
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	28.3	22.9
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	2,117.5	1,983.0
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,347.8	1,290.0
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	769.7	693.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	565.0	518.5
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	235.4	223.3
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	329.6	295.2
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	133.5	124.0
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	133.5	124.0
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	61.6	35.7
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	61.6	35.7
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	23.7	14.7
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	23.7	14.7
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	218.8	174.4
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	218.8	174.4
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	3.3	4.1
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	3.3	4.1
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,904.6	2,680.0
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,583.2	1,513.3
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,321.4	$ 1,166.7