Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended			65 Months Ended
	Mar. 31, 2019	Sep. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Jan. 27, 2015	Jan. 29, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchased During Period, Value		$ 99,100	$ 126,900	$ 136,100	$ 99,100	$ 1,070,000
Share Price			$ 16.31
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	82,250,000
Cancellation of Restricted Stock and Repurchase of Common Stock at Cost for Employee Tax Withholding, Value			$ 42,554	$ 52,336	$ 55,132
Maximum number of shares authorized			83,500,000
Document Period End Date			Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant			7,200,000
Stock Repurchased and Retired During Period, Shares		5,800,000	8,200,000	9,700,000	5,800,000	64,300,000
1995 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield			0.00%	0.00%	0.00%
Maximum number of shares authorized							20,000,000
Common stock price, percentage of the closing price on applicable offering commencement date								85.00%
Common stock price, percentage of closing price on applicable offering termination date							85.00%
Common stock reserved for issuance			3,900,000
Average risk-free interest rate			2.20%	2.00%	0.90%
Expected term (in years)			15 days	15 days	15 days
Weighted-average grant-date fair value per share			$ 3.76	$ 4.00	$ 3.84
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price for restricted units, vested			$ 0.001
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cancellation of Restricted Stock and Repurchase of Common Stock at Cost for Employee Tax Withholding, Value			$ 42,600
Weighted-average grant-date fair value per share			$ 16.52	$ 15.47	$ 16.31
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchased During Period, Value					$ 21,400
Stock Repurchased and Retired During Period, Shares					1,000,000.0