Stockholders' Equity - Schedule of Stockholders Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Class of Stock [Line Items]
Beginning Balance			$ 1,323,827
Beginning Balance(in shares)			59,890,885
Issuance of common stock, net of shares withheld for taxes			$ 8,822
Stock-based compensation			11,227
Other comprehensive income			1,279
Net income	$ 44,659	$ 41,126	55,733	$ 42,525
Ending balance	$ 1,400,888	$ 1,269,650	$ 1,400,888	$ 1,269,650
Ending Balance(in shares)	60,361,035	59,707,996	60,361,035	59,707,996
Common Stock [Member]
Class of Stock [Line Items]
Beginning Balance			$ 598
Beginning Balance(in shares)			59,890,885
Issuance of common stock, net of shares withheld for taxes			$ 5
Issuance of common stock, net of shares withheld for taxes (in shares)			470,150
Ending balance	$ 603		$ 603
Ending Balance(in shares)	60,361,035		60,361,035
Additional Paid-in Capital [Member]
Class of Stock [Line Items]
Beginning Balance			$ 694,564
Issuance of common stock, net of shares withheld for taxes			8,817
Stock-based compensation			11,227
Ending balance	$ 714,608		714,608
Retained Earnings [Member]
Class of Stock [Line Items]
Beginning Balance			647,322
Net income			55,733
Ending balance	703,055		703,055
Accumulated Other Comprehensive Loss [Member]
Class of Stock [Line Items]
Beginning Balance			(18,657)
Other comprehensive income			1,279
Ending balance	$ (17,378)		$ (17,378)