Restructuring Charges - Charges Taken Related to Accrued Restructuring Actions (Detail) - Restructuring Charges - USD ($) $ in Millions	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Fiscal 2019 charges:
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	$ 6.6
Severance payments	(7.7)
Other payments	(2.1)
Balance	5.9
Fiscal 2019 charges: | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
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]
Fiscal restructuring charges	1.2
Severance payments	(3.9)
Other payments	(0.5)
Balance	1.1
Fiscal 2019 charges: | Fiscal 2017 Actions
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	0.0
Severance payments	(0.8)
Other payments	0.0
Balance	0.0
Fiscal 2019 charges: | Previous Other Charges
Restructuring Reserve [Roll Forward]
Fiscal restructuring charges	1.4
Severance payments	0.0
Other payments	(1.6)
Balance	3.8
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 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: | Fiscal 2017 Actions
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 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 2017 charges:
Restructuring Reserve [Roll Forward]
Balance		11.5
Fiscal restructuring charges			$ 13.3
Severance payments			(6.6)
Other payments			(1.5)
Balance			11.5
Fiscal 2017 charges: | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
Balance		0.0
Fiscal restructuring charges			0.0
Severance payments			0.0
Other payments			0.0
Balance			0.0
Fiscal 2017 charges: | Fiscal 2018 Actions
Restructuring Reserve [Roll Forward]
Balance		0.0
Fiscal restructuring charges			0.0
Severance payments			0.0
Other payments			0.0
Balance			0.0
Fiscal 2017 charges: | Fiscal 2017 Actions
Restructuring Reserve [Roll Forward]
Balance		7.5
Fiscal restructuring charges			8.5
Severance payments			(1.0)
Other payments			0.0
Balance			7.5
Fiscal 2017 charges: | Previous Other Charges
Restructuring Reserve [Roll Forward]
Balance		$ 4.0
Fiscal restructuring charges			4.8
Severance payments			(5.6)
Other payments			(1.5)
Balance			4.0
Fiscal 2016 charges:
Restructuring Reserve [Roll Forward]
Balance			6.3
Fiscal 2016 charges: | Fiscal 2019 Actions
Restructuring Reserve [Roll Forward]
Balance			0.0
Fiscal 2016 charges: | Fiscal 2018 Actions
Restructuring Reserve [Roll Forward]
Balance			0.0
Fiscal 2016 charges: | Fiscal 2017 Actions
Restructuring Reserve [Roll Forward]
Balance			0.0
Fiscal 2016 charges: | Previous Other Charges
Restructuring Reserve [Roll Forward]
Balance			$ 6.3