Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,681.6	$ 3,616.7	$ 7,215.9	$ 7,106.5
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	1,190.3	1,150.2	2,291.1	2,149.0
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	921.7	869.8	1,699.2	1,587.5
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	268.6	280.4	591.9	561.5
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	481.4	554.4	947.4	1,055.3
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	345.6	379.7	638.2	707.0
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	135.8	174.7	309.2	348.3
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	432.0	475.3	954.4	935.7
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	244.1	264.3	521.8	509.3
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	187.9	211.0	432.6	426.4
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	23.0	24.1	51.3	47.0
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	23.0	24.1	51.3	47.0
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	2,126.7	2,204.0	4,244.2	4,187.0
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,511.4	1,513.8	2,859.2	2,803.8
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	615.3	690.2	1,385.0	1,383.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	494.6	488.2	1,059.6	1,006.7
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	210.3	230.6	445.7	453.9
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	284.3	257.6	613.9	552.8
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	106.2	120.3	239.7	244.3
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	106.2	120.3	239.7	244.3
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	44.8	47.3	106.4	83.0
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	44.8	47.3	106.4	83.0
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	20.6	16.8	44.3	31.5
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	20.6	16.8	44.3	31.5
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	171.6	184.4	390.4	358.8
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	171.6	184.4	390.4	358.8
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.8	3.7	6.1	7.8
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	2.8	3.7	6.1	7.8
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,795.7	2,880.3	5,700.3	5,560.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,721.7	1,744.4	3,304.9	3,257.7
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,074.0	$ 1,135.9	$ 2,395.4	$ 2,302.6