Debt - Additional Information (Detail)		3 Months Ended			12 Months Ended
	Sep. 24, 2019 USD ($) d	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Oct. 23, 2019 USD ($)	Aug. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Cash paid for interest		$ 7,615,000		$ 1,702,000
Available credit under lines of credit		$ 399,600,000	$ 399,600,000
Weighted average interest rate of total borrowings		1.80%	1.80%	1.90%
0.25% Convertible Senior Note Due 2022
Line of Credit Facility [Line Items]
Debt instrument, interest rate		0.25%	0.25%	0.25%
Aggregate principal amount							$ 345,000,000
Aggregate principal outstanding		$ 345,000,000	$ 345,000,000		$ 345,000,000
Debt instrument, conversion ratio		0.02125
Debt instrument conversion, conversion price per share (in usd per share) | $ / shares		$ 47.06	$ 47.06
Debt instrument conversion, If-converted value of notes		$ 297,400,000			$ 346,200,000
Effective interest rate		4.50%	4.50%		4.50%
Unamortized discount amount		$ 23,800,000	$ 23,800,000
Amortization period		3 years			3 years
0.50% Finisar Convertible Notes
Line of Credit Facility [Line Items]
Redemption price percentage	100.00%
Debt instrument, interest rate	0.50%	0.50%	0.50%
Aggregate principal amount	$ 560,100,000
Debt instrument, repurchase amount						$ 561,100,000
Aggregate principal outstanding		$ 14,900,000	$ 14,900,000
Bank of America, N.A.
Line of Credit Facility [Line Items]
Letter of credit sub-facility maximum borrowing capacity	25,000,000.0
Swing loan sub-facility maximum initial borrowing capacity	20,000,000.0
Senior Secured Credit Facility | Bank of America, N.A.
Line of Credit Facility [Line Items]
Aggregate principal amount	2,425,000,000
Term A Loan Facility
Line of Credit Facility [Line Items]
Proceeds from term loan	561,000,000.0
Aggregate principal outstanding		1,178,950,000	1,178,950,000		$ 1,194,463,000
Term A Loan Facility | Bank of America, N.A.
Line of Credit Facility [Line Items]
Aggregate principal amount	$ 1,255,000,000
Debt agreement term	5 years
Redemption price percentage	1.25%
Term B Loan Facility
Line of Credit Facility [Line Items]
Aggregate principal amount	$ 720,000,000
Debt agreement term	7 years
Cash paid for interest		600,000
Debt issuance costs			24,700,000
Aggregate principal outstanding		0	0		714,600,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Aggregate principal outstanding		$ 49,000,000	$ 49,000,000		$ 74,000,000
Revolving Credit Facility | Bank of America, N.A.
Line of Credit Facility [Line Items]
Aggregate principal amount	$ 450,000,000
Debt agreement term	5 years
Senior Credit Facilities | Fifth Fiscal Quarter Through Eighth Fiscal Quarter After Closing Date
Line of Credit Facility [Line Items]
Leverage ratio	4.50
Senior Credit Facilities | Subsequent Fiscal Quarter
Line of Credit Facility [Line Items]
Leverage ratio	4.00
Senior Credit Facilities | Minimum
Line of Credit Facility [Line Items]
Interest coverage ratio	3.00
Senior Credit Facilities | Maximum | First Four Fiscal Quarters After Closing Date
Line of Credit Facility [Line Items]
Leverage ratio	5.00
Senior Credit Facilities | Federal Funds Rate
Line of Credit Facility [Line Items]
Debt instrument, rate added on variable rate	0.50%
Senior Credit Facilities | Eurodollar
Line of Credit Facility [Line Items]
Debt instrument, rate added on variable rate	1.00%
Senior Credit Facilities | 0.25% Convertible Senior Note Due 2022 | I I V I Notes
Line of Credit Facility [Line Items]
Number of days prior to maturity of outstanding | d	120
Debt instrument, interest rate	0.25%