Revenue - Revenues by Product (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Total product revenue	$ 9,835.6	$ 10,173.4	$ 10,981.7
TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue	1,012.5	1,443.9	1,951.9
VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue	576.3	553.4	410.4
Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	1,588.8	1,997.3	2,362.3
AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue	811.0	973.5	1,208.7
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue	294.7	331.9	357.4
Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	1,105.7	1,305.4	1,566.1
TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	1,876.9	2,030.9	2,063.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	90.5	96.6	105.2
Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue	4,661.9	5,430.2	6,096.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	1,741.2	1,793.5	1,905.1
QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue	5.9	0.0	0.0
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue	55.9	0.0	0.0
Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue	1,803.0	1,793.5	1,905.1
BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue	438.8	441.0	498.3
IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	222.1	224.5	233.4
FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	77.4	81.3	99.4
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue	31.7	4.3	0.0
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	770.0	751.1	831.1
Other
Disaggregation of Revenue [Line Items]
Total product revenue	11.8	13.0	14.0
Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	7,246.7	7,987.8	8,846.9
United States | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue	263.1	417.7	680.6
United States | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue	512.1	521.3	408.9
United States | Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	775.2	939.0	1,089.5
United States | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue	536.7	649.2	830.2
United States | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue	126.2	148.4	152.9
United States | Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	662.9	797.6	983.1
United States | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	997.9	1,123.4	1,142.2
United States | FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
United States | Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue	2,436.0	2,860.0	3,214.8
United States | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	610.5	600.2	587.9
United States | QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue	5.8	0.0	0.0
United States | SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue	55.9	0.0	0.0
United States | Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue	672.2	600.2	587.9
United States | BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
United States | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
United States | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
United States | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue	29.2	4.3	0.0
United States | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	29.2	4.3	0.0
United States | Other
Disaggregation of Revenue [Line Items]
Total product revenue	4.0	4.8	3.0
United States | Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	3,141.4	3,469.3	3,805.7
Rest of World | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue	749.4	1,026.2	1,271.3
Rest of World | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue	64.2	32.1	1.5
Rest of World | Total Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	813.6	1,058.3	1,272.8
Rest of World | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue	274.3	324.3	378.5
Rest of World | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue	168.5	183.5	204.5
Rest of World | Total Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	442.8	507.8	583.0
Rest of World | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	879.0	907.5	920.9
Rest of World | FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	90.5	96.6	105.2
Rest of World | Subtotal: Multiple Sclerosis
Disaggregation of Revenue [Line Items]
Total product revenue	2,225.9	2,570.2	2,881.9
Rest of World | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	1,130.7	1,193.3	1,317.2
Rest of World | QALSODY
Disaggregation of Revenue [Line Items]
Total product revenue	0.1	0.0	0.0
Rest of World | SKYCLARYS
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
Rest of World | Subtotal: Rare Disease
Disaggregation of Revenue [Line Items]
Total product revenue	1,130.8	1,193.3	1,317.2
Rest of World | BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue	438.8	441.0	498.3
Rest of World | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	222.1	224.5	233.4
Rest of World | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	77.4	81.3	99.4
Rest of World | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue	2.5	0.0	0.0
Rest of World | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	740.8	746.8	831.1
Rest of World | Other
Disaggregation of Revenue [Line Items]
Total product revenue	7.8	8.2	11.0
Rest of World | Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	$ 4,105.3	$ 4,518.5	$ 5,041.2