Debt - Additional Information (Detail)		3 Months Ended	12 Months Ended
	Sep. 24, 2019 USD ($) d	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Sep. 23, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2018	Aug. 31, 2017 USD ($)
Line Of Credit Facility [Line Items]
Available credit under lines of credit		$ 290,000,000.0			$ 211,900,000
Weighted average interest rate of total borrowings		1.90%				1.50%
0.25% Convertible Senior Note Due 2022
Line Of Credit Facility [Line Items]
Debt instrument, interest rate		0.25%			0.25%	0.25%
Unamortized discount amount		$ 35,500,000
Amortization period		3 years
Debt instrument maturity date		Sep. 01, 2022
Debt instrument conversion, conversion price per share | $ / shares		$ 47.06
Aggregate principal amount							$ 345,000,000
Debt instrument conversion, shares issued per $1,000 principal amount		21.25
Debt instrument conversion, principal amount of each note converted		$ 1,000
Debt instrument conversion, If-converted value of notes		$ 258,100,000	$ 268,000,000.0
Effective interest rate		4.50%				4.50%
0.50% Convertible Senior Notes Due 2036
Line Of Credit Facility [Line Items]
Debt instrument, interest rate	0.50%
Zero Point Five Zero Finisar Convertible Notes
Line Of Credit Facility [Line Items]
Percentage of obligated to repay the outstanding principal amount	100.00%
Debt instrument, interest rate	0.50%	0.50%
Debt instrument redemption period date	Dec. 22, 2021
Debt instrument maturity date	Dec. 15, 2036
Debt instrument maturity date description	The Finisar Notes will mature on December 15, 2036
Debt instrument payment terms description	Interest on the Finisar Notes accrues at 0.50% per annum, paid semi-annually, in arrears, on June 15 and December 15 of each year.
Debt instrument frequency of periodic payment	semi-annually
Aggregate principal amount	$ 560,100,000
Debt instrument, repurchase amount	$ 561,100,000
Debt instrument, repurchase date	Oct. 23, 2019
Line of credit, outstanding				$ 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
Term A Loan Facility | Bank of America, N.A.
Line Of Credit Facility [Line Items]
Aggregate principal amount	$ 1,255,000,000
Debt Instrument, Term	5 years
Percentage of obligated to repay the outstanding principal amount	1.25%
Term B Loan Facility | Bank of America, N.A.
Line Of Credit Facility [Line Items]
Aggregate principal amount	$ 720,000,000,000.0
Debt Instrument, Term	7 years
Percentage of obligated to repay the outstanding principal amount	0.25%
Revolving Credit Facility | Bank of America, N.A.
Line Of Credit Facility [Line Items]
Aggregate principal amount	$ 450,000,000,000.0
Debt Instrument, Term	5 years
New Senior Credit Facilities
Line Of Credit Facility [Line Items]
Debt issuance costs incurred		$ 69,800,000
Debt issuance costs capitalized		65,800,000
Debt issuance expense		4,000,000.0
Unamortized discount amount		$ 65,600,000
New Senior Credit Facilities | Fifth Fiscal Quarter Through Eighth Fiscal Quarter After Closing Date
Line Of Credit Facility [Line Items]
Leverage ratio	4.50
New Senior Credit Facilities | Subsequent Fiscal Quarter
Line Of Credit Facility [Line Items]
Leverage ratio	4.00
New Senior Credit Facilities | Minimum
Line Of Credit Facility [Line Items]
Interest coverage ratio	3.00
Amortization period		5 years
New Senior Credit Facilities | Maximum
Line Of Credit Facility [Line Items]
Amortization period		7 years
New Senior Credit Facilities | Maximum | First Four Fiscal Quarters After Closing Date
Line Of Credit Facility [Line Items]
Leverage ratio	5.00
New Senior Credit Facilities | Federal Funds Rate
Line Of Credit Facility [Line Items]
Debt instrument, rate added on variable rate	0.50%
New Senior Credit Facilities | Eurodollar
Line Of Credit Facility [Line Items]
Debt instrument, rate added on variable rate	1.00%
New 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%
Debt instrument maturity year	2022