Business Combinations (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,065,379,000		$ 1,065,379,000		$ 555,965,000
F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Revenue	568,272,000	$ 546,324,000	1,671,875,000	$ 1,609,341,000
Acquired Tangible Assets	44,494,000		44,494,000
Cash Acquired from Acquisition	29,911,000		29,911,000
Payments to Acquire Businesses, Gross			643,208,000
Business Acquisition, Transaction Costs	19,000,000.0		19,000,000.0
Business Combination, Acquired Receivable, Fair Value	7,306,000		7,306,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	4,214,000		4,214,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	1,822,000		1,822,000
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets	10,357,000		10,357,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	89,300,000		$ 89,300,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			8 years 1 month 6 days
Goodwill	509,414,000		$ 509,414,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	652,324,000		652,324,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	(1,081,000)		(1,081,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue	(4,000,000)		(4,000,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	(4,035,000)		(4,035,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(9,116,000)		(9,116,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	643,208,000		643,208,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount	437,400,000		437,400,000
Business Acquisition, Pro Forma Information, Description	100,329,000	$ 110,800,000	324,074,000	$ 267,742,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual			5,100,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual			5,900,000
Developed Technology Rights [Member] | F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	62,500,000		$ 62,500,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			7 years
Customer Relationships [Member] | F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	12,000,000		$ 12,000,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			15 years
Trade Names [Member] | F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	14,500,000		$ 14,500,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			7 years
Non-competition Agreements [Member] | F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	300,000		$ 300,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			2 years
Acquisition-related Costs [Member] | F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Retention costs			$ 21,800,000
Business Acquisition, Transaction Costs	1,000,000.0		1,000,000.0
General and Administrative Expense [Member] | F5 Networks Acquisition of Nginx [Member]
Business Acquisition [Line Items]
Business Acquisition, Transaction Costs	$ 1,000,000.0		$ 1,000,000.0