Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,804.1	$ 2,534.7	$ 8,017.9	$ 7,611.2
Product revenue, net
Disaggregation of Revenue [Line Items]
Total revenue	2,088.7	1,846.9	5,757.4	5,452.1
Product revenue, net \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,265.0	935.3	3,218.7	2,630.2
Product revenue, net \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	823.7	911.6	2,538.7	2,821.9
MS Product Revenues
Disaggregation of Revenue [Line Items]
Total revenue	919.3	1,061.5	2,840.1	3,121.7
MS Product Revenues \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	584.4	642.9	1,764.8	1,790.7
MS Product Revenues \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	334.9	418.6	1,075.3	1,331.0
Fumarate
Disaggregation of Revenue [Line Items]
Total revenue	274.0	382.8	849.9	1,133.6
Fumarate \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	197.9	234.2	587.1	626.3
Fumarate \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	76.1	148.6	262.8	507.3
TECFIDERA
Disaggregation of Revenue [Line Items]
Total revenue	82.0	168.2	282.4	567.9
TECFIDERA \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	31.2	44.6	95.0	131.6
TECFIDERA \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	50.8	123.6	187.4	436.3
VUMERITY
Disaggregation of Revenue [Line Items]
Total revenue	192.0	214.6	567.5	565.7
VUMERITY \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	166.7	189.6	492.1	494.7
VUMERITY \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	25.3	25.0	75.4	71.0
Interferon
Disaggregation of Revenue [Line Items]
Total revenue	208.2	246.9	660.8	719.9
Interferon \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	133.2	161.2	412.1	443.9
Interferon \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	75.0	85.7	248.7	276.0
AVONEX
Disaggregation of Revenue [Line Items]
Total revenue	156.6	188.5	489.8	533.0
AVONEX \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	110.0	133.4	339.4	363.7
AVONEX \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	46.6	55.1	150.4	169.3
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total revenue	51.6	58.4	171.0	186.9
PLEGRIDY \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	23.2	27.8	72.7	80.2
PLEGRIDY \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	28.4	30.6	98.3	106.7
TYSABRI
Disaggregation of Revenue [Line Items]
Total revenue	437.1	431.8	1,329.4	1,267.9
TYSABRI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	253.3	247.5	765.6	720.5
TYSABRI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	183.8	184.3	563.8	547.4
FAMPYRA
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.3
FAMPYRA \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.0
FAMPYRA \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.3
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Total revenue	676.3	533.3	1,835.2	1,639.6
Rare Disease Product Revenue \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	354.4	235.1	873.8	700.9
Rare Disease Product Revenue \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	321.9	298.2	961.4	938.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Total revenue	460.6	374.0	1,236.5	1,190.6
SPINRAZA \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	246.4	153.2	592.9	456.9
SPINRAZA \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	214.2	220.8	643.6	733.7
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total revenue	178.2	132.9	496.8	387.1
SKYCLARYS \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	97.2	74.6	251.3	221.7
SKYCLARYS \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	81.0	58.3	245.5	165.4
QALSODY
Disaggregation of Revenue [Line Items]
Total revenue	37.5	26.4	101.9	61.9
QALSODY \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	10.8	7.3	29.6	22.3
QALSODY \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	26.7	19.1	72.3	39.6
Specialized Immunology
Disaggregation of Revenue [Line Items]
Total revenue	242.8	0.0	370.6	0.0
Specialized Immunology \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	242.8	0.0	370.6	0.0
Specialized Immunology \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0	0.0	0.0
EMPAVELI
Disaggregation of Revenue [Line Items]
Total revenue	51.5	0.0	81.9	0.0
EMPAVELI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	51.5		81.9	0.0
EMPAVELI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0		0.0	0.0
SYFOVRE
Disaggregation of Revenue [Line Items]
Total revenue	191.3	0.0	288.7	0.0
SYFOVRE \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	191.3	0.0	288.7	0.0
SYFOVRE \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0	0.0	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Total revenue	165.9	196.8	500.9	559.3
Biosimilars \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	2.0	0.1	8.8
Biosimilars \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	165.9	194.8	500.8	550.5
BENEPALI
Disaggregation of Revenue [Line Items]
Total revenue	117.3	121.9	346.2	345.3
BENEPALI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0	0.0	0.0
BENEPALI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	117.3	121.9	346.2	345.3
IMRALDI
Disaggregation of Revenue [Line Items]
Total revenue	42.0	52.6	129.4	146.7
IMRALDI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0	0.0	0.0
IMRALDI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	42.0	52.6	129.4	146.7
FLIXABI
Disaggregation of Revenue [Line Items]
Total revenue	6.4	15.3	25.0	42.7
FLIXABI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0	0.0	0.0
FLIXABI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	6.4	15.3	25.0	42.7
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total revenue	0.2	7.0	0.3	24.5
BYOOVIZ \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	2.0	0.1	8.7
BYOOVIZ \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.2	5.0	0.2	15.8
TOFIDENCE
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.1
TOFIDENCE \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.1
TOFIDENCE \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Total revenue	84.4	55.3	210.6	131.5
Other \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	83.4	55.3	209.4	129.8
Other \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	1.0	0.0	1.2	1.7
ZURZUVAE
Disaggregation of Revenue [Line Items]
Total revenue	84.4	55.3	210.6	129.4
ZURZUVAE \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	83.4	55.3	209.4	129.4
ZURZUVAE \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	$ 1.0	$ 0.0	1.2	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total revenue			0.0	2.1
FUMADERM And ADUHELM \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue			0.0	0.4
FUMADERM And ADUHELM \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue			$ 0.0	$ 1.7