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

Amount of increase (decrease) in the value of right to a contingent consideration asset.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Fair value portion of notes payable.

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