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		87,495	$ 87,495			87,495
Other comprehensive income, net of tax:
Net change in net unrealized gain (loss) on securities available for sale		6,713	6,713				6,713
Comprehensive income		94,208	94,208
Common stock issued in business acquisition		107,478		35	107,443
Stock-based compensation, net of tax		2,680			2,680
Common stock cash dividends		(23,201)				(23,201)
Ending balance at Jun. 30, 2016		1,640,511		587	1,311,510	319,383	9,031
Other comprehensive income, net of tax:
Cumulative effect adjustment	[1]	263			751	(488)
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		107,125	107,125			107,125
Other comprehensive income, net of tax:
Net change in net unrealized gain (loss) on securities available for sale		(11,037)	(11,037)				(11,037)
Comprehensive income		96,088	$ 96,088
Stock-based compensation, net of tax		4,963			4,963
Common stock cash dividends		(31,722)				(31,722)
Ending balance at Jun. 30, 2017		$ 1,732,983		$ 587	$ 1,318,061	$ 419,838	$ (5,503)

[1] Cumulative effect adjustment relates to adoption of ASU 2016-09.