Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,508.5	$ 2,778.9	$ 7,629.4	$ 8,247.9
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,340.3	1,556.0	4,161.4	4,568.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	707.9	833.4	2,170.7	2,410.9
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	632.4	722.6	1,990.7	2,157.6
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	476.8	619.5	1,549.4	1,750.9
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	220.4	299.9	713.3	805.1
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	256.4	319.6	836.1	945.8
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	339.0	498.6	1,146.8	1,465.4
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	92.5	179.2	330.3	520.1
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	246.5	319.4	816.5	945.3
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	137.8	120.9	402.6	285.5
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	127.9	120.7	383.0	285.0
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	9.9	0.2	19.6	0.5
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	336.0	387.5	995.8	1,188.4
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	214.5	252.4	608.0	751.6
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	121.5	135.1	387.8	436.8
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	255.1	301.3	743.4	923.3
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	174.8	213.2	493.8	636.4
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	80.3	88.1	249.6	286.9
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	80.9	86.2	252.4	265.1
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	39.7	39.2	114.2	115.2
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	41.2	47.0	138.2	149.9
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	505.5	522.8	1,542.5	1,550.4
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	273.0	281.1	849.4	854.2
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	232.5	241.7	693.1	696.2
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	22.0	26.2	73.7	78.8
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	22.0	26.2	73.7	78.8
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	431.1	444.1	1,334.7	1,464.4
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	140.2	139.8	443.3	437.8
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	290.9	304.3	891.4	1,026.6
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	187.6	202.8	576.3	610.2
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.7	0.0	1.2	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	186.9	202.8	575.1	610.2
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	110.2	120.8	340.7	363.9
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	110.2	120.8	340.7	363.9
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.7	57.4	172.4	170.9
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	57.7	57.4	172.4	170.9
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	19.0	24.6	62.0	75.4
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	19.0	24.6	62.0	75.4
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	0.7	0.0	1.2	0.0
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.7	0.0	1.2	0.0
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	1.5	2.5	6.4	8.3
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	1.5	2.5	6.4	8.3
ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	1.6	0.3	4.5	2.0
ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1.6	0.3	4.5	2.0
ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,962.1	2,205.7	6,083.3	6,653.4
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	850.4	973.5	2,619.7	2,850.7
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,111.7	$ 1,232.2	$ 3,463.6	$ 3,802.7