Revenue - Revenues by Product (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total product revenue	$ 10,173.4	$ 10,981.7	$ 13,444.6
Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	1,997.3	2,362.3	3,905.4
Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	1,305.4	1,566.1	1,877.5
TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	2,030.9	2,063.1	1,946.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	96.6	105.2	103.1
Subtotal: MS
Disaggregation of Revenue [Line Items]
Total product revenue	5,430.2	6,096.7	7,832.1
SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	1,793.5	1,905.1	2,052.1
ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue	4.8	3.0	0.0
BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue	441.0	498.3	481.6
IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	224.5	233.4	216.3
FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	81.3	99.4	97.9
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	751.1	831.1	795.8
FUMADERM
Disaggregation of Revenue [Line Items]
Total product revenue	8.2	11.0	12.2
Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	7,987.8	8,846.9	10,692.2
TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue	1,443.9	1,951.9	3,841.1
VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue	553.4	410.4	64.3
AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue	973.5	1,208.7	1,491.9
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue	331.9	357.4	385.6
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue	4.3	0.0	0.0
United States | Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	939.0	1,089.5	2,742.0
United States | Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	797.6	983.1	1,273.5
United States | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	1,123.4	1,142.2	1,096.8
United States | FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
United States | Subtotal: MS
Disaggregation of Revenue [Line Items]
Total product revenue	2,860.0	3,214.8	5,112.3
United States | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	600.2	587.9	787.8
United States | ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue	4.8	3.0	0.0
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 | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	4.3	0.0	0.0
United States | FUMADERM
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
United States | Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	3,469.3	3,805.7	5,900.1
United States | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue	417.7	680.6	2,677.7
United States | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue	521.3	408.9	64.3
United States | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue	649.2	830.2	1,083.4
United States | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue	148.4	152.9	190.1
United States | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue	4.3	0.0	0.0
Rest of World | Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	1,058.3	1,272.8	1,163.4
Rest of World | Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	507.8	583.0	604.0
Rest of World | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	907.5	920.9	849.3
Rest of World | FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	96.6	105.2	103.1
Rest of World | Subtotal: MS
Disaggregation of Revenue [Line Items]
Total product revenue	2,570.2	2,881.9	2,719.8
Rest of World | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	1,193.3	1,317.2	1,264.3
Rest of World | ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue	0.0	0.0	0.0
Rest of World | BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue	441.0	498.3	481.6
Rest of World | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	224.5	233.4	216.3
Rest of World | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	81.3	99.4	97.9
Rest of World | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	746.8	831.1	795.8
Rest of World | FUMADERM
Disaggregation of Revenue [Line Items]
Total product revenue	8.2	11.0	12.2
Rest of World | Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	4,518.5	5,041.2	4,792.1
Rest of World | TECFIDERA
Disaggregation of Revenue [Line Items]
Total product revenue	1,026.2	1,271.3	1,163.4
Rest of World | VUMERITY
Disaggregation of Revenue [Line Items]
Total product revenue	32.1	1.5	0.0
Rest of World | AVONEX
Disaggregation of Revenue [Line Items]
Total product revenue	324.3	378.5	408.5
Rest of World | PLEGRIDY
Disaggregation of Revenue [Line Items]
Total product revenue	183.5	204.5	195.5
Rest of World | BYOOVIZ
Disaggregation of Revenue [Line Items]
Total product revenue	$ 0.0	$ 0.0	$ 0.0