Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,464.9	$ 2,456.0	$ 4,755.4	$ 4,919.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,899.6	1,845.8	3,611.5	3,609.1
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	846.7	801.0	1,592.8	1,502.4
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1,052.9	1,044.8	2,018.7	2,106.7
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,149.8	1,209.3	2,225.7	2,334.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	582.4	636.7	1,085.6	1,182.8
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	567.4	572.6	1,140.1	1,151.7
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	418.0	400.4	799.8	783.1
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	188.3	196.8	337.9	365.0
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	229.7	203.6	461.9	418.1
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	252.2	254.2	506.5	528.7
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	44.1	66.5	87.8	141.2
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	208.1	187.7	418.7	387.5
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	165.8	146.2	293.3	254.4
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	144.2	130.3	250.1	223.8
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	21.6	15.9	43.2	30.6
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	250.9	302.4	494.5	548.0
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	145.4	180.0	285.2	312.5
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	105.5	122.4	209.3	235.5
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	182.8	220.3	361.3	392.7
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	117.2	145.9	228.4	248.5
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	65.6	74.4	132.9	144.2
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	68.1	82.1	133.2	155.3
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	28.2	34.1	56.8	64.0
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	39.9	48.0	76.4	91.3
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	462.2	483.1	893.5	955.9
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	248.7	259.9	462.5	505.3
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	213.5	223.2	431.0	450.6
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	18.7	23.4	37.9	47.5
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	18.7	23.4	37.9	47.5
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Product revenues	534.1	438.0	958.0	881.3
Rare Disease Product Revenue | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	237.5	156.7	463.4	303.4
Rare Disease Product Revenue | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	296.6	281.3	494.6	577.9
QALSODY
Disaggregation of Revenue [Line Items]
Product revenues	5.0		9.6	0.9
QALSODY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	4.6		9.0	0.9
QALSODY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.4		0.6	0.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	429.1	437.1	770.4	880.4
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	157.3	155.8	305.8	302.5
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	271.8	281.3	464.6	577.9
Completed technology for SKYCLARYS (U.S.)
Disaggregation of Revenue [Line Items]
Product revenues	100.0		178.0	0.0
Completed technology for SKYCLARYS (U.S.) | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	75.6		148.6	0.0
Completed technology for SKYCLARYS (U.S.) | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	24.4		29.4	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	198.1	195.1	395.0	387.5
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	11.1	7.0	14.8	15.2
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	187.0	188.1	380.2	372.3
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	117.3	109.2	236.0	218.2
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	117.3	109.2	236.0	218.2
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	53.2	58.8	108.0	113.2
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	53.2	58.8	108.0	113.2
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	13.1	20.1	30.9	40.5
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	13.1	20.1	30.9	40.5
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	13.7	7.0	19.3	15.6
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	10.3	7.0	14.0	15.2
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	3.4	0.0	5.3	0.4
TOFIDENCE
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0.0	0.8	0.0
TOFIDENCE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0.0	0.8	0.0
TOFIDENCE | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Product revenues	17.6	3.4	32.8	5.8
Other | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	15.7	0.6	29.0	1.0
Other | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1.9	2.8	3.8	4.8
ZURZUVAE
Disaggregation of Revenue [Line Items]
Product revenues	14.9	0.0	27.3	0.0
ZURZUVAE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	14.9	0.0	27.3	0.0
ZURZUVAE | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	2.7	3.4	5.5	5.8
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0.6	1.7	1.0
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1.9	$ 2.8	$ 3.8	$ 4.8