Restructuring Charges - Charges Taken Related to Restructuring Actions (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 28, 2020	Sep. 26, 2020	Sep. 28, 2019	Sep. 29, 2018
Restructuring Cost and Reserve [Line Items]
Divestiture charges	$ 1,900,000
Fiscal 2018 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions				$ 11,700,000
Facility closure costs				2,500,000
Fiscal restructuring charges				14,200,000
Fiscal 2019 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions			$ 5,400,000
Facility closure costs			1,200,000
Fiscal restructuring charges			6,600,000
Fiscal 2020 | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions		$ 13,400,000
Divestiture charges		1,900,000
Fiscal restructuring charges		15,300,000
Fiscal 2020 charges | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Fiscal restructuring charges		15,300,000
Fiscal 2019 Actions | Fiscal 2018 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions				0
Facility closure costs				0
Fiscal restructuring charges				0
Fiscal 2019 Actions | Fiscal 2019 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions			4,000,000.0
Facility closure costs			0
Fiscal restructuring charges			4,000,000.0
Fiscal 2019 Actions | Fiscal 2020 | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions		300,000
Divestiture charges		0
Fiscal restructuring charges		300,000
Fiscal 2019 Actions | Fiscal 2020 charges | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Fiscal restructuring charges		300,000
Fiscal 2018 Actions | Fiscal 2018 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions				11,700,000
Facility closure costs				900,000
Fiscal restructuring charges				12,600,000
Fiscal 2018 Actions | Fiscal 2019 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions			1,400,000
Facility closure costs			(200,000)
Fiscal restructuring charges			1,200,000
Fiscal 2018 Actions | Fiscal 2020 | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions		(100,000)
Divestiture charges		0
Fiscal restructuring charges		(100,000)
Fiscal 2018 Actions | Fiscal 2020 charges | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Fiscal restructuring charges		(100,000)
Other action [Member] | Fiscal 2018 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions				0
Facility closure costs				1,600,000
Fiscal restructuring charges				1,600,000
Other action [Member] | Fiscal 2019 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions			0
Facility closure costs			1,400,000
Fiscal restructuring charges			1,400,000
Other action [Member] | Fiscal 2020 | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions		0
Divestiture charges		0
Fiscal restructuring charges		0
Fiscal 2020 Action | Fiscal 2018 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions				0
Facility closure costs				0
Fiscal restructuring charges				$ 0
Fiscal 2020 Action | Fiscal 2019 charges: | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions			0
Facility closure costs			0
Fiscal restructuring charges			$ 0
Fiscal 2020 Action | Fiscal 2020 | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Workforce reductions		13,200,000
Divestiture charges		1,900,000
Fiscal restructuring charges		15,100,000
Fiscal 2020 Action | Fiscal 2020 charges | Restructuring Charges
Restructuring Cost and Reserve [Line Items]
Fiscal restructuring charges		$ 15,100,000