Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended			65 Months Ended
	Sep. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Apr. 29, 2015	Apr. 29, 2013
Stockholders Equity Note [Line Items]
Stock Repurchase Program, Number of Shares Authorized, Value						$ 500,000	$ 500,000
Preferred stock, shares authorized		40,000,000
Preferred stock, par value		$ 0.001
Stock Repurchase Program, Remaining Value of Shares Authorized to be Repurchased		$ 430,400
Stock Repurchased and Retired During Period, Shares	5,800,000	8,200,000	9,700,000	5,800,000	64,300,000
Stock Repurchased During Period, Value	$ 99,100	$ 126,900	$ 136,100	$ 99,100	$ 1,070,000
Document Period End Date		Sep. 30, 2019
Issuance of common stock in connection with business and asset acquisitions				$ 16,347
Stock Issued During Period, Shares, New Issues			175,000
Stock Issued During Period, Value, Other			$ 2,900
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		3,314	2,963	1,932,286
Payments for Repurchase of Common Stock		$ 126,938	$ 136,090	$ 99,077
Series A Preferred Stock [Member]
Stockholders Equity Note [Line Items]
Preferred stock, par value		$ 0.001
Preferred Stock, Number of Votes, Per Share		$ 1,000
Series B preferred stock
Stockholders Equity Note [Line Items]
Preferred stock, shares authorized		15,000,000
Preferred Stock, Conversion Rate		1
Preferred Stock, Liquidation Preference Per Share		$ 1.30
Preferred stock, non-cumulative dividends		$ 0.05
Chief Executive Officer [Member]
Stockholders Equity Note [Line Items]
Stock Repurchased and Retired During Period, Shares				1,000,000.0
Stock Repurchased During Period, Value				$ 21,400
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				800,000
Stock Option, Net-settled share equivalent				350,351
Chief Executive Officer [Member] | Common Stock
Stockholders Equity Note [Line Items]
Stock Repurchased and Retired During Period, Shares				649,649
Common Stock
Stockholders Equity Note [Line Items]
Stock Repurchased and Retired During Period, Shares		8,160,000	9,698,000	5,797,000
Issuance of common stock in connection with business and asset acquisitions (in shares)				1,019,000
Issuance of common stock in connection with business and asset acquisitions				$ 1
Other Acquisitions [Member]
Stockholders Equity Note [Line Items]
Issuance of common stock in connection with business and asset acquisitions (in shares)			844,108
Issuance of common stock in connection with business and asset acquisitions			$ 13,400