Fair Value Measurements (Details 1) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Notes Payable	$ 6,286.4	$ 6,281.0
Debt Instrument, Fair Value Disclosure	5,198.5	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	1,689.6	1,699.9
Notes Payable	1,746.2	1,744.7
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Notes payable, fair value	972.7	1,033.2
Notes Payable	1,100.5	1,100.3
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value	1,196.2	1,219.0
Notes Payable	1,493.6	1,492.9
3.250% Senior Notes, Due February 15, 2051
Debt Instrument [Line Items]
Notes payable, fair value	430.8	469.1
Notes Payable	471.9	469.3
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Notes payable, fair value	909.2	989.0
Notes Payable	$ 1,474.2	$ 1,473.8