Stock-Based Compensation Stock-Based Compensation - Summary of Weighted Average Grant-Date Fair Value, Shares Issued and Total Stock-Based Compensation Expense Recognized Related to the Plan (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (in millions)	$ 141,212	$ 142,909	$ 142,901
1995 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value per share	$ 3.76	$ 4.00	$ 3.84
Total shares issued (in millions)	1.2	1.3	1.3
Total stock-based compensation expense (in millions)	$ 4,500	$ 5,200	$ 4,900