Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,530.3	$ 2,508.5	$ 7,449.3	$ 7,629.4
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,159.0	1,340.3	3,493.5	4,161.4
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	631.8	707.9	1,814.6	2,170.7
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	527.2	632.4	1,678.9	1,990.7
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	405.0	476.8	1,188.1	1,549.4
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	206.9	220.4	571.9	713.3
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	198.1	256.4	616.2	836.1
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	239.5	339.0	768.2	1,146.8
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	58.1	92.5	199.3	330.3
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	181.4	246.5	568.9	816.5
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	165.5	137.8	419.9	402.6
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	148.8	127.9	372.6	383.0
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	16.7	9.9	47.3	19.6
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	277.7	336.0	825.7	995.8
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	180.1	214.5	492.6	608.0
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	97.6	121.5	333.1	387.8
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	212.2	255.1	604.9	743.4
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	148.7	174.8	397.2	493.8
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	63.5	80.3	207.7	249.6
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	65.5	80.9	220.8	252.4
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	31.4	39.7	95.4	114.2
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	34.1	41.2	125.4	138.2
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	456.3	505.5	1,412.2	1,542.5
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	244.8	273.0	750.1	849.4
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	211.5	232.5	662.1	693.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	20.0	22.0	67.5	73.7
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	20.0	22.0	67.5	73.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	448.2	431.1	1,328.6	1,334.7
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	150.5	140.2	453.0	443.3
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	297.7	290.9	875.6	891.4
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	194.3	187.6	581.8	576.3
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	6.1	0.7	21.3	1.2
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	188.2	186.9	560.5	575.1
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	112.8	110.2	331.0	340.7
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	112.8	110.2	331.0	340.7
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	54.4	57.7	167.6	172.4
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	54.4	57.7	167.6	172.4
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	20.2	19.0	60.7	62.0
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	20.2	19.0	60.7	62.0
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	6.9	0.7	22.5	1.2
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	6.1	0.7	21.3	1.2
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0.0	1.2	0.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,805.2	1,962.1	5,414.3	6,083.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	790.9	850.4	2,293.3	2,619.7
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1,014.3	1,111.7	3,121.0	3,463.6
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	3.7	3.1	10.4	10.9
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	2.5	1.6	4.4	4.5
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1.2	1.5	6.0	6.4
Spinal Muscular Atrophy
Disaggregation of Revenue [Line Items]
Product revenues	448.2	431.1	1,328.6	1,334.7
Spinal Muscular Atrophy | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	150.5	140.2	453.0	443.3
Spinal Muscular Atrophy | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 297.7	$ 290.9	$ 875.6	$ 891.4