Revenue - Revenues by Product (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Total product revenue, net	$ 9,890.6	$ 9,675.9	$ 9,835.6
Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,426.5	1,595.1	1,588.8
TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue, net	679.7	967.1	1,012.5
VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue, net	746.8	628.0	576.3
Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue, net	945.6	968.0	1,105.7
AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue, net	695.5	707.5	811.0
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue, net	250.1	260.5	294.7
Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue, net	4,038.9	4,349.8	4,661.9
TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,665.4	1,715.0	1,876.9
FAMPYRA(1)
Disaggregation of Revenue [Line Items]
Total product revenue, net	1.4	71.7	90.5
Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue, net	2,154.2	1,988.1	1,803.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,546.8	1,573.2	1,741.2
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue, net	520.5	382.5	55.9
QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue, net	86.9	32.4	5.9
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue, net	729.1	793.1	770.0
BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue, net	453.2	479.1	438.8
IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue, net	190.2	213.1	222.1
FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue, net	52.6	63.2	77.4
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue, net	32.4	36.6	31.7
TOFIDENCE
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.7	1.1	0.0
Other
Disaggregation of Revenue [Line Items]
Total product revenue, net	197.2	82.5	11.8
ZURZUVAE
Disaggregation of Revenue [Line Items]
Total product revenue, net	195.1	72.2	1.6
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue, net	2.1	10.3	10.2
Total product revenue, net
Disaggregation of Revenue [Line Items]
Total product revenue, net	7,119.4	7,213.5	7,246.7
United States | Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue, net	819.7	707.8	775.2
United States | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue, net	168.5	169.2	263.1
United States | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue, net	651.2	538.6	512.1
United States | Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue, net	587.8	562.7	662.9
United States | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue, net	482.9	451.3	536.7
United States | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue, net	104.9	111.4	126.2
United States | Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue, net	2,372.5	2,190.5	2,436.0
United States | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue, net	965.0	920.0	997.9
United States | FAMPYRA(1)
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.0	0.0	0.0
United States | Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue, net	966.2	947.7	672.2
United States | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue, net	625.5	625.7	610.5
United States | SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue, net	310.6	301.1	55.9
United States | QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue, net	30.1	20.9	5.8
United States | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue, net	13.7	24.1	29.2
United States | BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.0	0.0	0.0
United States | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.0	0.0	0.0
United States | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.0	0.0	0.0
United States | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue, net	13.0	23.0	29.2
United States | TOFIDENCE
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.7	1.1	0.0
United States | Other
Disaggregation of Revenue [Line Items]
Total product revenue, net	195.5	75.0	4.0
United States | ZURZUVAE
Disaggregation of Revenue [Line Items]
Total product revenue, net	195.1	72.2	1.6
United States | FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.4	2.8	2.4
United States | Total product revenue, net
Disaggregation of Revenue [Line Items]
Total product revenue, net	3,547.9	3,237.3	3,141.4
Rest of World | Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue, net	606.8	887.3	813.6
Rest of World | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue, net	511.2	797.9	749.4
Rest of World | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue, net	95.6	89.4	64.2
Rest of World | Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue, net	357.8	405.3	442.8
Rest of World | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue, net	212.6	256.2	274.3
Rest of World | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue, net	145.2	149.1	168.5
Rest of World | Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,666.4	2,159.3	2,225.9
Rest of World | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue, net	700.4	795.0	879.0
Rest of World | FAMPYRA(1)
Disaggregation of Revenue [Line Items]
Total product revenue, net	1.4	71.7	90.5
Rest of World | Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,188.0	1,040.4	1,130.8
Rest of World | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue, net	921.3	947.5	1,130.7
Rest of World | SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue, net	209.9	81.4	0.0
Rest of World | QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue, net	56.8	11.5	0.1
Rest of World | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue, net	715.4	769.0	740.8
Rest of World | BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue, net	453.2	479.1	438.8
Rest of World | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue, net	190.2	213.1	222.1
Rest of World | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue, net	52.6	63.2	77.4
Rest of World | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue, net	19.4	13.6	2.5
Rest of World | TOFIDENCE
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.0	0.0	0.0
Rest of World | Other
Disaggregation of Revenue [Line Items]
Total product revenue, net	1.7	7.5	7.8
Rest of World | ZURZUVAE
Disaggregation of Revenue [Line Items]
Total product revenue, net	0.0	0.0	0.0
Rest of World | FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue, net	1.7	7.5	7.8
Rest of World | Total product revenue, net
Disaggregation of Revenue [Line Items]
Total product revenue, net	$ 3,571.5	$ 3,976.2	$ 4,105.3