v3.26.1
Revenues - Revenues by product (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Disaggregation of Revenue [Line Items]        
Total revenue $ 2,736.0 $ 2,645.5 $ 5,213.8 $ 5,076.5
Product revenue, net        
Disaggregation of Revenue [Line Items]        
Total revenue 1,916.4 1,878.7 3,668.7 3,605.2
Product revenue, net | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 1,114.5 941.1 1,953.7 1,694.9
Product revenue, net | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 801.9 937.6 1,715.0 1,910.3
MS Product Revenues        
Disaggregation of Revenue [Line Items]        
Total revenue 963.3 1,107.2 1,920.8 2,060.2
MS Product Revenues | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 621.0 657.4 1,180.4 1,147.8
MS Product Revenues | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 342.3 449.8 740.4 912.4
Fumarate        
Disaggregation of Revenue [Line Items]        
Total revenue 287.4 405.9 575.9 750.8
Fumarate | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 204.4 235.2 389.2 392.1
Fumarate | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 83.0 170.7 186.7 358.7
TECFIDERA        
Disaggregation of Revenue [Line Items]        
Total revenue 90.9 193.6 200.4 399.7
TECFIDERA | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 32.4 47.2 63.8 87.0
TECFIDERA | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 58.5 146.4 136.6 312.7
VUMERITY        
Disaggregation of Revenue [Line Items]        
Total revenue 196.5 212.3 375.5 351.1
VUMERITY | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 172.0 188.0 325.4 305.1
VUMERITY | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 24.5 24.3 50.1 46.0
Interferon        
Disaggregation of Revenue [Line Items]        
Total revenue 225.1 246.7 452.6 473.0
Interferon | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 146.1 150.0 278.9 282.7
Interferon | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 79.0 96.7 173.7 190.3
AVONEX        
Disaggregation of Revenue [Line Items]        
Total revenue 170.0 177.7 333.2 344.5
AVONEX | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 120.9 121.7 229.4 230.3
AVONEX | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 49.1 56.0 103.8 114.2
PLEGRIDY        
Disaggregation of Revenue [Line Items]        
Total revenue 55.1 69.0 119.4 128.5
PLEGRIDY | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 25.2 28.3 49.5 52.4
PLEGRIDY | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 29.9 40.7 69.9 76.1
TYSABRI        
Disaggregation of Revenue [Line Items]        
Total revenue 450.8 454.6 892.3 836.1
TYSABRI | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 270.5 272.2 512.3 473.0
TYSABRI | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 180.3 182.4 380.0 363.1
FAMPYRA        
Disaggregation of Revenue [Line Items]        
Total revenue     0.0 0.3
FAMPYRA | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue     0.0 0.0
FAMPYRA | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue     0.0 0.3
Rare Disease Product Revenue        
Disaggregation of Revenue [Line Items]        
Total revenue 601.7 543.0 1,158.9 1,106.3
Rare Disease Product Revenue | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 294.9 234.8 519.4 465.8
Rare Disease Product Revenue | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 306.8 308.2 639.5 640.5
SPINRAZA        
Disaggregation of Revenue [Line Items]        
Total revenue 401.9 392.7 775.9 816.6
SPINRAZA | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 204.3 149.3 346.5 303.7
SPINRAZA | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 197.6 243.4 429.4 512.9
SKYCLARYS        
Disaggregation of Revenue [Line Items]        
Total revenue 167.9 130.3 318.6 254.2
SKYCLARYS | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 82.3 78.0 154.1 147.1
SKYCLARYS | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 85.6 52.3 164.5 107.1
QALSODY        
Disaggregation of Revenue [Line Items]        
Total revenue 31.9 20.0 64.4 35.5
QALSODY | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 8.3 7.5 18.8 15.0
QALSODY | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 23.6 12.5 45.6 20.5
Specialized Immunology        
Disaggregation of Revenue [Line Items]        
Total revenue 127.8 0.0 127.8 0.0
Specialized Immunology | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 127.8 0.0 127.8 0.0
Specialized Immunology | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.0 0.0 0.0
EMPAVELI        
Disaggregation of Revenue [Line Items]        
Total revenue 30.4 0.0 30.4 0.0
EMPAVELI | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 30.4   30.4 0.0
EMPAVELI | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0   0.0 0.0
SYFOVRE        
Disaggregation of Revenue [Line Items]        
Total revenue 97.4 0.0 97.4 0.0
SYFOVRE | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 97.4 0.0 97.4 0.0
SYFOVRE | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.0 0.0 0.0
Biosimilars        
Disaggregation of Revenue [Line Items]        
Total revenue 152.8 181.7 335.0 362.5
Biosimilars | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 0.1 2.5 0.1 6.8
Biosimilars | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 152.7 179.2 334.9 355.7
BENEPALI        
Disaggregation of Revenue [Line Items]        
Total revenue 106.8 112.1 228.9 223.4
BENEPALI | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.0 0.0 0.0
BENEPALI | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 106.8 112.1 228.9 223.4
IMRALDI        
Disaggregation of Revenue [Line Items]        
Total revenue 37.8 46.7 87.4 94.1
IMRALDI | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.0 0.0 0.0
IMRALDI | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 37.8 46.7 87.4 94.1
FLIXABI        
Disaggregation of Revenue [Line Items]        
Total revenue 8.1 14.3 18.6 27.4
FLIXABI | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.0 0.0 0.0
FLIXABI | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 8.1 14.3 18.6 27.4
BYOOVIZ        
Disaggregation of Revenue [Line Items]        
Total revenue 0.1 8.6 0.1 17.5
BYOOVIZ | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 0.1 2.5 0.1 6.7
BYOOVIZ | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 6.1 0.0 10.8
TOFIDENCE        
Disaggregation of Revenue [Line Items]        
Total revenue     0.0 0.1
TOFIDENCE | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue     0.0 0.1
TOFIDENCE | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue     0.0 0.0
Other        
Disaggregation of Revenue [Line Items]        
Total revenue 70.8 46.8 126.2 76.2
Other | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 70.7 46.4 126.0 74.5
Other | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 0.1 0.4 0.2 1.7
ZURZUVAE        
Disaggregation of Revenue [Line Items]        
Total revenue 70.8 46.4 126.2 74.1
ZURZUVAE | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 70.7 46.4 126.0 74.1
ZURZUVAE | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue 0.1 0.0 0.2 0.0
FUMADERM And ADUHELM        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.4 0.0 2.1
FUMADERM And ADUHELM | U.S.        
Disaggregation of Revenue [Line Items]        
Total revenue 0.0 0.0 0.0 0.4
FUMADERM And ADUHELM | Rest of World        
Disaggregation of Revenue [Line Items]        
Total revenue $ 0.0 $ 0.4 $ 0.0 $ 1.7