Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Oct. 01, 2019	Dec. 29, 2018	Sep. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Percent of standalone selling price for subscriptions that contains software license	55.00%
Percent of standalone selling price for subscriptions that contains support	45.00%
Contract with customer, refund liability	$ 34,500,000	$ 22,900,000
Impairment charges	500,000
Non-marketable equity investments	8,900,000	9,400,000
Allowance for doubtful accounts receivable	500,000	700,000	$ 600,000
Allowance for credit loss, writeoff	200,000	200,000	1,000,000.0
Bad debt expense including general and administrative expense	$ 0.0	300,000	500,000
Lease expiration date	2037
Development costs for software	$ 0	0	0
Advertising expense	3,800,000	3,600,000	2,900,000
Cumulative translation adjustment gains (loss)	(69,100,000)	(91,200,000)
Pension benefits, before tax	(37,200,000)	(34,900,000)
Accumulated other comprehensive (income) loss, defined benefit plan, after tax	26,400,000	24,800,000
AOCI, Debt Securities, Available-for-sale, Adjustment, before tax	300,000	100,000
AOCI, Debt Securities, Available-for-sale, Adjustment, after Tax	300,000	100,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Net Investment Hedges, Before Tax	(8,200,000)	6,800,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Net Investment Hedges, Effect Net of Tax	(8,200,000)	$ 5,100,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		1.0
Operating lease, liability	215,023,000
Operating right-of-use lease assets	149,933,000	$ 0
Total stockholders’ equity	1,438,248,000	1,201,998,000	874,589,000			$ 885,436,000
Purchased Software
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets acquired	$ 11,500,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Percent of standalone selling price for subscriptions	5.00%
Minimum | Computer Hardware And Software
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	3 years
Minimum | Furniture And Fixtures
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Percent of standalone selling price for subscriptions	50.00%
Maximum | Purchased Software
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	16 years
Maximum | Customer Lists
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	12 years
Maximum | Trademarks And Trade Names
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	12 years
Maximum | Computer Hardware And Software
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	5 years
Maximum | Furniture And Fixtures
Summary Of Significant Accounting Policies [Line Items]
Amortization period (in years)	7 years
Accumulated Deficit
Summary Of Significant Accounting Policies [Line Items]
Total stockholders’ equity	$ (62,267,000)	$ (191,390,000)	$ (599,409,000)			$ (650,840,000)
Accounting Standards Update 2016-16 | Accumulated Deficit
Summary Of Significant Accounting Policies [Line Items]
Total stockholders’ equity					$ 72,300,000
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease, liability				$ 224,000,000.0
Operating right-of-use lease assets				167,900,000
Accounting Standards Update 2016-02 | Accumulated Deficit | Scheduling of Reversal of Temporary Tax Differences | Cumulative Effect, Period of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Total stockholders’ equity				$ (1,600,000)
Not Designated as Hedging Instrument | Foreign Exchange Forward
Summary Of Significant Accounting Policies [Line Items]
Derivative maturity	3 months
Net Investment Hedging | Designated as Hedging Instrument | Foreign Exchange Forward
Summary Of Significant Accounting Policies [Line Items]
Derivative maturity	3 months