Restructuring Charges - Charges Taken Related to Accrued Restructuring Actions (Detail) - Restructuring Charges - USD ($) $ in Millions	12 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 29, 2018
Fiscal 2020 charges
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	$ 15.3
Stock-based compensation	(7.5)
Severance payments	(5.9)
Other payments	(3.3)
Balance	4.5
Fiscal 2020 charges | Fiscal 2020 Action
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	15.1
Stock-based compensation	(7.5)
Severance payments	(4.4)
Other payments	0.5
Balance	3.7
Fiscal 2020 charges | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	0.3
Stock-based compensation	0.0
Severance payments	(1.3)
Other payments	0.0
Balance	0.0
Fiscal 2020 charges | Fiscal 2018 Actions
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	(0.1)
Stock-based compensation	0.0
Severance payments	(0.2)
Other payments	0.0
Balance	0.8
Fiscal 2020 charges | Previous Other Charges
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	0.0
Stock-based compensation	0.0
Severance payments	0.0
Other payments	(3.8)
Balance	0.0
Fiscal 2020 charges | Fiscal 2017 Actions [Member]
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	0.0
Stock-based compensation	0.0
Severance payments	0.0
Other payments	0.0
Balance	0.0
Fiscal 2019 charges:
Restructuring Reserve [Roll Forward]
Balance	5.9
Fiscal restructuring charges		$ 6.6
Severance payments		(7.7)
Other payments		(2.1)
Balance		5.9
Fiscal 2019 charges: | Fiscal 2020 Action
Restructuring Reserve [Roll Forward]
Balance	0.0
Fiscal restructuring charges		0.0
Severance payments		0.0
Other payments		0.0
Balance		0.0
Fiscal 2019 charges: | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
Balance	1.0
Fiscal restructuring charges		4.0
Severance payments		(3.0)
Other payments		0.0
Balance		1.0
Fiscal 2019 charges: | Fiscal 2018 Actions
Restructuring Reserve [Roll Forward]
Balance	1.1
Fiscal restructuring charges		1.2
Severance payments		(3.9)
Other payments		(0.5)
Balance		1.1
Fiscal 2019 charges: | Previous Other Charges
Restructuring Reserve [Roll Forward]
Balance	3.8
Fiscal restructuring charges		1.4
Severance payments		0.0
Other payments		(1.6)
Balance		3.8
Fiscal 2019 charges: | Fiscal 2017 Actions [Member]
Restructuring Reserve [Roll Forward]
Balance	$ 0.0
Fiscal restructuring charges		0.0
Severance payments		(0.8)
Other payments		0.0
Balance		0.0
Fiscal 2018 charges:
Restructuring Reserve [Roll Forward]
Balance		9.1
Fiscal restructuring charges			$ 14.2
Stock-based compensation			(1.3)
Severance payments			(13.7)
Other payments			(1.6)
Balance			9.1
Fiscal 2018 charges: | Fiscal 2020 Action
Restructuring Reserve [Roll Forward]
Balance		0.0
Fiscal restructuring charges			0.0
Stock-based compensation			0.0
Severance payments			0.0
Other payments			0.0
Balance			0.0
Fiscal 2018 charges: | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
Balance		0.0
Fiscal restructuring charges			0.0
Stock-based compensation			0.0
Severance payments			0.0
Other payments			0.0
Balance			0.0
Fiscal 2018 charges: | Fiscal 2018 Actions
Restructuring Reserve [Roll Forward]
Balance		4.3
Fiscal restructuring charges			12.6
Stock-based compensation			(1.3)
Severance payments			(6.8)
Other payments			(0.2)
Balance			4.3
Fiscal 2018 charges: | Previous Other Charges
Restructuring Reserve [Roll Forward]
Balance		4.0
Fiscal restructuring charges			1.6
Stock-based compensation			0.0
Severance payments			(0.2)
Other payments			(1.4)
Balance			4.0
Fiscal 2018 charges: | Fiscal 2017 Actions [Member]
Restructuring Reserve [Roll Forward]
Balance		$ 0.8
Fiscal restructuring charges			0.0
Stock-based compensation			0.0
Severance payments			(6.7)
Other payments			0.0
Balance			0.8
Fiscal 2017 charges:
Restructuring Reserve [Roll Forward]
Balance			11.5
Fiscal 2017 charges: | Fiscal 2020 Action
Restructuring Reserve [Roll Forward]
Balance			0.0
Fiscal 2017 charges: | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
Balance			0.0
Fiscal 2017 charges: | Fiscal 2018 Actions
Restructuring Reserve [Roll Forward]
Balance			0.0
Fiscal 2017 charges: | Previous Other Charges
Restructuring Reserve [Roll Forward]
Balance			4.0
Fiscal 2017 charges: | Fiscal 2017 Actions [Member]
Restructuring Reserve [Roll Forward]
Balance			$ 7.5