Consolidated Statement of Stockholders' Equity (Unaudited) - 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, 2015	$ 1,459,346		$ 552	$ 1,201,387	$ 255,089	$ 2,318
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30,461	$ 30,461			30,461
Other comprehensive income, net of tax:
Net change in net unrealized gain (loss) on securities available for sale	(7,607)	(7,607)				(7,607)
Comprehensive income	22,854	22,854
Stock-based compensation, net of tax	1,049			1,049
Common stock cash dividends	(7,733)				(7,733)
Ending balance at Dec. 31, 2015	1,475,516		552	1,202,436	277,817	(5,289)
Beginning balance at Sep. 30, 2016	1,663,391		587	1,312,347	344,923	5,534
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36,903	36,903			36,903
Other comprehensive income, net of tax:
Net change in net unrealized gain (loss) on securities available for sale	(13,310)	(13,310)				(13,310)
Comprehensive income	23,593	$ 23,593
Stock-based compensation, net of tax	1,635			1,635
Common stock cash dividends	(9,981)				(9,981)
Ending balance at Dec. 31, 2016	$ 1,678,638		$ 587	$ 1,313,982	$ 371,845	$ (7,776)