Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Tax credits	$ 252.8		$ 112.6
Inventory, other reserves and accruals	203.7		202.8
Intangibles, net	1,153.9		1,370.3
Neurimmune's tax basis in ADUHELM	0.0		470.3
IRC Section 174 capitalized research and development	570.8		271.8
Net operating loss	1,700.4		1,845.9
Share-based compensation	36.1		37.2
Other	293.3		280.7
Valuation allowance	(1,278.7)	$ (85.0)	(2,003.3)
Total deferred tax assets	2,932.3		2,588.3
Deferred tax liabilities:
Purchased inventory valuation step-up and intangible assets	(1,257.4)		(76.1)
Samsung Bioepis investment installments	(35.5)		(138.0)
GILTI	(1,136.9)		(1,002.0)
Tax credits	0.0		(228.7)
Depreciation, amortization and other	(215.7)		(251.8)
Total deferred tax liabilities	$ (2,645.5)		$ (1,696.6)

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