Income Taxes - Activity of Company's Unrecognized Income Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 25, 2021	Sep. 26, 2020
Income Tax Disclosure [Abstract]
Balance at beginning of fiscal year	$ 197.1	$ 101.6
Tax positions related to current year:
Additions	8.0	109.6
Reductions	0.0	0.0
Tax positions related to prior years:
Additions related to change in estimate	7.9	1.5
Reductions	(0.3)	(0.7)
Payments	0.0	0.0
Lapses in statutes of limitations	(1.7)	(15.6)
Acquired tax positions:
Additions related to reserves acquired from acquisitions	1.8	0.7
Balance as of the end of the fiscal year	$ 212.8	$ 197.1

Tax positions related to current year.

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Tax positions related to prior years.

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Unrecognized tax benefits increases.

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Amount of unrecognized tax benefits.

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from acquisitions.

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