Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Comprehensive Income	Common Stock Par Value	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income / (Loss)
Beginning balance at Sep. 30, 2013	$ 1,417,214		$ 579	$ 1,260,124	$ 163,592	$ (7,081)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	104,952	$ 104,952			104,952
Other comprehensive income, net of tax:
Net change in net unrealized (loss) on securities available for sale	924	924				924
Comprehensive income	105,876	105,876
Cash dividends:
Common stock, $0.56, $0.36, and $1.76 per share in 2016, 2015 and 2014, respectively	(102,000)				(102,000)
Ending balance at Sep. 30, 2014	1,421,090		579	1,260,124	166,544	(6,157)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	109,065	109,065			109,065
Other comprehensive income, net of tax:
Net change in net unrealized (loss) on securities available for sale	8,475	8,475				8,475
Comprehensive income	117,540	117,540
Stock-based compensation, net of tax	1,236			1,236
Common stock repurchased	(60,000)		(27)	(59,973)
Cash dividends:
Common stock, $0.56, $0.36, and $1.76 per share in 2016, 2015 and 2014, respectively	(20,520)				(20,520)
Ending balance at Sep. 30, 2015	1,459,346		552	1,201,387	255,089	2,318
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	121,253	121,253			121,253
Other comprehensive income, net of tax:
Net change in net unrealized (loss) on securities available for sale	3,216	3,216				3,216
Comprehensive income	124,469	$ 124,469
Stock-based compensation, net of tax	3,517			3,517
Common stock issued in business acquisition	107,478		35	107,443
Cash dividends:
Common stock, $0.56, $0.36, and $1.76 per share in 2016, 2015 and 2014, respectively	(31,419)				(31,419)
Ending balance at Sep. 30, 2016	$ 1,663,391		$ 587	$ 1,312,347	$ 344,923	$ 5,534