Revenues - Revenues by Product (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Total product revenue	$ 10,981.7	$ 13,444.6	$ 14,377.9
Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	2,362.3	3,905.4	4,438.2
Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	1,566.1	1,877.5	2,101.8
TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	2,063.1	1,946.1	1,892.2
FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	105.2	103.1	97.1
Subtotal: MS
Disaggregation of Revenue [Line Items]
Total product revenue	6,096.7	7,832.1	8,529.3
SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	1,905.1	2,052.1	2,097.0
ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue	3.0	0.0	0.0
BENEPALI
Disaggregation of Revenue [Line Items]
Total product revenue	498.3	481.6	486.2
IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	233.4	216.3	184.0
FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	99.4	97.9	68.1
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	831.1	795.8	738.3
FUMADERM
Disaggregation of Revenue [Line Items]
Total product revenue	11.0	12.2	15.2
Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	8,846.9	10,692.2	11,379.8
United States | Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	1,089.5	2,742.0	3,312.0
United States | Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	983.1	1,273.5	1,426.6
United States | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	1,142.2	1,096.8	1,041.8
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	3,214.8	5,112.3	5,780.4
United States | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	587.9	787.8	933.4
United States | ADUHELM
Disaggregation of Revenue [Line Items]
Total product revenue	3.0	0.0	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	0.0	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	3,805.7	5,900.1	6,713.8
Rest of World | Fumarate
Disaggregation of Revenue [Line Items]
Total product revenue	1,272.8	1,163.4	1,126.2
Rest of World | Interferon
Disaggregation of Revenue [Line Items]
Total product revenue	583.0	604.0	675.2
Rest of World | TYSABRI
Disaggregation of Revenue [Line Items]
Total product revenue	920.9	849.3	850.4
Rest of World | FAMPYRA
Disaggregation of Revenue [Line Items]
Total product revenue	105.2	103.1	97.1
Rest of World | Subtotal: MS
Disaggregation of Revenue [Line Items]
Total product revenue	2,881.9	2,719.8	2,748.9
Rest of World | SPINRAZA
Disaggregation of Revenue [Line Items]
Total product revenue	1,317.2	1,264.3	1,163.6
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	498.3	481.6	486.2
Rest of World | IMRALDI
Disaggregation of Revenue [Line Items]
Total product revenue	233.4	216.3	184.0
Rest of World | FLIXABI
Disaggregation of Revenue [Line Items]
Total product revenue	99.4	97.9	68.1
Rest of World | Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Total product revenue	831.1	795.8	738.3
Rest of World | FUMADERM
Disaggregation of Revenue [Line Items]
Total product revenue	11.0	12.2	15.2
Rest of World | Total product revenue
Disaggregation of Revenue [Line Items]
Total product revenue	$ 5,041.2	$ 4,792.1	$ 4,666.0