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Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Tax credits	$ 112.6		$ 121.0
Inventory, other reserves and accruals	202.8		199.4
Intangibles, net	1,370.3		1,477.5
Neurimmune's tax basis in ADUHELM	470.3		475.8
IRC Section 174 capitalized research and development	271.8		0.0
Net operating loss	1,845.9		1,973.0
Share-based compensation	37.2		31.7
Other	280.7		208.8
Valuation allowance	(2,003.3)	$ (85.0)	(1,961.3)
Total deferred tax assets	2,588.3		2,525.9
Deferred tax liabilities:
Purchased intangible assets	(76.1)		(256.6)
Samsung Bioepis investment installments	(138.0)		0.0
GILTI	(1,002.0)		(1,037.6)
Tax credits	(228.7)		(260.2)
Depreciation, amortization and other	(251.8)		(250.9)
Total deferred tax liabilities	$ (1,696.6)		$ (1,805.3)

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