Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Tax credits	$ 121.0	$ 113.4
Inventory, other reserves and accruals	199.4	165.9
Intangibles, net	1,477.5	1,546.0
Neurimmune's tax basis in ADUHELM	475.8	0.0
Net operating loss	1,973.0	2,080.3
Share-based compensation	31.7	23.3
Other	208.8	103.1
Valuation allowance	(1,961.3)	(1,753.9)
Total deferred tax assets	2,525.9	2,278.1
Deferred tax liabilities:
Purchased intangible assets	(256.6)	(396.2)
GILTI	(1,037.6)	(1,143.7)
Tax credits	(260.2)	(174.6)
Depreciation, amortization and other	(250.9)	(227.0)
Total deferred tax liabilities	$ (1,805.3)	$ (1,941.5)