Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)		3 Months Ended		6 Months Ended
	Oct. 28, 2016	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Apr. 20, 2016	Jan. 21, 2015
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Number of Reportable Segments				1
Revenue Recognition
VSOE percentage which constitutes substantial majority of transactions priced within a narrow range				80.00%
Range of VSOE to median sales price				15.00%
Stock-Based Compensation
Share-based compensation expense		$ 43,900,000	$ 41,800,000	$ 90,500,000	$ 80,000,000
Unrecognized stock-based compensation cost		191,800,000		$ 191,800,000
Unrecognized stock-based compensation cost, period for recognition				2 years
Common Stock Repurchase
Stock Repurchase Program, Number Of Shares Repurchased And Retired				2,141,502
Treasury Stock Acquired, Average Cost Per Share				$ 140.11
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 473,700,000		$ 473,700,000
Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program	1,242,170
Minimum [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life				3 years
Revenue Recognition
Domestic accounts receivable terms of payment				30 days
International accounts receivable terms of payment				30 days
Maximum [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life				10 years
Revenue Recognition
Domestic accounts receivable terms of payment				45 days
International accounts receivable terms of payment				120 days
October Twenty Six Two Thousand Ten Program [Member]
Common Stock Repurchase
Stock Repurchase Program, Authorized Amount						$ 1,000,000,000	$ 2,400,000,000
Annual Equity Program [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Percentage of the aggregate number of RSUs granted that vest in equal quarterly increments				50.00%
Portion of RSU grant subject to Company achieving specified quarterly revenue and EBITDA goals				50.00%
Percentage of quarterly performance stock grant based on achieving quarterly revenue goal				70.00%
Percentage of quarterly revenue goal to be achieved for performance stock grant				80.00%
Percentage of quarterly performance stock grant based on achieving EBITDA goal				30.00%
Percentage Of Quarterly Ebitda Goal To Be Achieved For Performance Stock Grant				80.00%
Threshold percentage of targeted goals above which quarterly performance stock grant is paid linearly				80.00%
Percentage of over-achievement threshold to which the goals are entitled				100.00%
Percentage Of Over-Achievement To Which Quarterly Performance Based Vesting Cannot Exceed				200.00%
Annual Equity Program [Member] | November 1, 2016 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program				115,347
Annual equity awards program vesting period				4 years
Annual Equity Program [Member] | November 2, 2015 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program				145,508
Annual equity awards program vesting period				4 years
Annual Equity Program [Member] | November 1, 2014 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program				171,575
Annual equity awards program vesting period				4 years
Annual Equity Program [Member] | November 1, 2013 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program				231,320
Annual equity awards program vesting period				4 years
Software and Software Development Costs [Member] | Minimum [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life				3 years
Software and Software Development Costs [Member] | Maximum [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life				5 years