Consolidated Statements Of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative effect of adopting ASU [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Cumulative effect of adopting ASU [Member]	Unearned Compensation ESOP [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative effect of adopting ASU [Member]	AOCI [Member]
Balance at Sep. 30, 2017	$ 1,368,313		$ 1,382	$ 1,167,368		$ (37,995)	$ 234,640		$ 2,918
Balance (Accounting Standards Update 2016-09 [Member]) at Sep. 30, 2017		$ 0			$ 19			$ (19)
Net income	98,927						98,927
Other comprehensive income (loss), net of tax	755								755
Reclassification of certain tax effects related to adopting Accounting Standards Update ("ASU") 2018-02	0						(667)		667
Capital City Bancshares, Inc. ("CCB") acquisition	39,113		30	39,083
ESOP activity	2,193			541		1,652
Stock-based compensation	372			372
Stock options exercised	261			261
Cash dividends to stockholders	(118,312)						(118,312)
Balance at Sep. 30, 2018	1,391,622		1,412	1,207,644		(36,343)	214,569		4,340
Balance (Accounting Standards Update 2014-09 [Member]) at Sep. 30, 2018		394						394
Net income	94,243						94,243
Other comprehensive income (loss), net of tax	(19,239)								(19,239)
ESOP activity	2,200			549		1,651
Restricted stock activity, net	(2)		1	(3)
Stock-based compensation	552			552
Stock options exercised	1,485		1	1,484
Cash dividends to stockholders	(134,929)						(134,929)
Balance at Sep. 30, 2019	1,336,326		1,414	1,210,226		(34,692)	174,277		(14,899)
Balance (Accounting Standards Update 2016-02 [Member]) at Sep. 30, 2019		$ 88						$ 88
Net income	64,540						64,540
Other comprehensive income (loss), net of tax	(1,606)								(1,606)
ESOP activity	1,988			336		1,652
Restricted stock activity, net	(19)			(19)
Stock-based compensation	570			570
Repurchase of common stock	(23,804)		(26)	(21,897)			(1,881)
Stock options exercised	638		1	637
Cash dividends to stockholders	(93,862)						(93,862)
Balance at Sep. 30, 2020	$ 1,284,859		$ 1,389	$ 1,189,853		$ (33,040)	$ 143,162		$ (16,505)