Revenue - Revenues by Product (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Total product revenue, net	$ 9,675.9	$ 9,835.6	$ 10,173.4
Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,595.1	1,588.8	1,997.3
TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue, net	967.1	1,012.5	1,443.9
VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue, net	628.0	576.3	553.4
Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue, net	968.0	1,105.7	1,305.4
AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue, net	707.5	811.0	973.5
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue, net	260.5	294.7	331.9
Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue, net	4,349.8	4,661.9	5,430.2
TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,715.0	1,876.9	2,030.9
FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue, net	71.7	90.5	96.6
Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,988.1	1,803.0	1,793.5
SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,573.2	1,741.2	1,793.5
QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue, net	32.4	5.9	0.0
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue, net	382.5	55.9	0.0
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue, net	793.1	770.0	751.1
BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue, net	479.1	438.8	441.0
IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue, net	213.1	222.1	224.5
FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue, net	63.2	77.4	81.3
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue, net	36.6	31.7	4.3
TOFIDENCE
Disaggregation of Revenue [Line Items]
Total product revenue, net	1.1	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Total product revenue, net	82.5	11.8	13.0
ZURZUVAE
Disaggregation of Revenue [Line Items]
Total product revenue, net	72.2	1.6	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue, net	10.3	10.2	13.0
Total product revenue, net
Disaggregation of Revenue [Line Items]
Total product revenue, net	7,213.5	7,246.7	7,987.8
United States | Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue, net	707.8	775.2	939.0
United States | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue, net	169.2	263.1	417.7
United States | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue, net	538.6	512.1	521.3
United States | Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue, net	562.7	662.9	797.6
United States | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue, net	451.3	536.7	649.2
United States | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue, net	111.4	126.2	148.4
United States | Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue, net	2,190.5	2,436.0	2,860.0
United States | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue, net	920.0	997.9	1,123.4
United States | FAMPYRA
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	947.7	672.2	600.2
United States | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue, net	625.7	610.5	600.2
United States | QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue, net	20.9	5.8	0.0
United States | SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue, net	301.1	55.9	0.0
United States | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue, net	24.1	29.2	4.3
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	23.0	29.2	4.3
United States | TOFIDENCE
Disaggregation of Revenue [Line Items]
Total product revenue, net	1.1	0.0	0.0
United States | Other
Disaggregation of Revenue [Line Items]
Total product revenue, net	75.0	4.0	4.8
United States | ZURZUVAE
Disaggregation of Revenue [Line Items]
Total product revenue, net	72.2	1.6	0.0
United States | FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue, net	2.8	2.4	4.8
United States | Total product revenue, net
Disaggregation of Revenue [Line Items]
Total product revenue, net	3,237.3	3,141.4	3,469.3
Rest of World | Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue, net	887.3	813.6	1,058.3
Rest of World | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue, net	797.9	749.4	1,026.2
Rest of World | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue, net	89.4	64.2	32.1
Rest of World | Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue, net	405.3	442.8	507.8
Rest of World | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue, net	256.2	274.3	324.3
Rest of World | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue, net	149.1	168.5	183.5
Rest of World | Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue, net	2,159.3	2,225.9	2,570.2
Rest of World | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue, net	795.0	879.0	907.5
Rest of World | FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue, net	71.7	90.5	96.6
Rest of World | Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue, net	1,040.4	1,130.8	1,193.3
Rest of World | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue, net	947.5	1,130.7	1,193.3
Rest of World | QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue, net	11.5	0.1	0.0
Rest of World | SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue, net	81.4	0.0	0.0
Rest of World | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue, net	769.0	740.8	746.8
Rest of World | BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue, net	479.1	438.8	441.0
Rest of World | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue, net	213.1	222.1	224.5
Rest of World | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue, net	63.2	77.4	81.3
Rest of World | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue, net	13.6	2.5	0.0
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	7.5	7.8	8.2
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	7.5	7.8	8.2
Rest of World | Total product revenue, net
Disaggregation of Revenue [Line Items]
Total product revenue, net	$ 3,976.2	$ 4,105.3	$ 4,518.5