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Fair Value Measurements (Details 1) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2023 | Dec. 31, 2022
Debt Instrument [Line Items] |  |
Notes Payable | $ 6,282.7 | $ 6,281.0
Debt Instrument, Fair Value Disclosure | 5,640.9 | 5,410.2
BIIB054 |  |
Debt Instrument [Line Items] |  |
Reduction in contingent consideration | 195.4 |
4.050% Senior Notes due 2025 |  |
Debt Instrument [Line Items] |  |
Notes payable, fair value | 1,711.6 | 1,699.9
Notes Payable | 1,745.2 | 1,744.7
2.250% Senior Notes due May 1, 2030 |  |
Debt Instrument [Line Items] |  |
Notes payable, fair value | 1,130.2 | 1,033.2
Notes Payable | 1,100.4 | 1,100.3
5.200% Senior Notes due 2045 |  |
Debt Instrument [Line Items] |  |
Notes payable, fair value | 1,262.2 | 1,219.0
Notes Payable | 1,493.1 | 1,492.9
3.250% Senior Notes, Due February 15, 2051 |  |
Debt Instrument [Line Items] |  |
Notes payable, fair value | 493.2 | 469.1
Notes Payable | 470.1 | 469.3
3.150% Senior Notes due May 1, 2050 |  |
Debt Instrument [Line Items] |  |
Notes payable, fair value | 1,043.7 | 989.0
Notes Payable | $ 1,473.9 | $ 1,473.8

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