Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,376.1	$ 3,600.1	$ 10,592.0	$ 10,706.6
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	967.5	1,122.4	3,258.6	3,271.4
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	684.2	842.0	2,383.4	2,429.5
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	283.3	280.4	875.2	841.9
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	474.1	530.0	1,421.5	1,585.3
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	327.3	360.3	965.5	1,067.3
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	146.8	169.7	456.0	518.0
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	516.5	483.6	1,470.9	1,419.3
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	304.2	263.0	826.0	772.3
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	212.3	220.6	644.9	647.0
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.8	24.2	78.1	71.2
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	26.8	24.2	78.1	71.2
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,984.9	2,160.2	6,229.1	6,347.2
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,315.7	1,465.3	4,174.9	4,269.1
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	669.2	694.9	2,054.2	2,078.1
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	494.4	547.1	1,554.0	1,553.8
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	182.5	236.7	628.2	690.6
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	311.9	310.4	925.8	863.2
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	124.2	115.9	363.9	360.2
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	124.2	115.9	363.9	360.2
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	56.2	49.3	162.6	132.3
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	56.2	49.3	162.6	132.3
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	27.5	18.4	71.8	49.9
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	27.5	18.4	71.8	49.9
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	207.9	183.6	598.3	542.4
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	207.9	183.6	598.3	542.4
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	3.1	3.8	9.2	11.6
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	3.1	3.8	9.2	11.6
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,690.3	2,894.7	8,390.6	8,455.0
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,498.2	1,702.0	4,803.1	4,959.7
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,192.1	$ 1,192.7	$ 3,587.5	$ 3,495.3