Consolidated Statements Of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative effect of adopting ASU [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Unearned Compensation ESOP [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative effect of adopting ASU [Member]	AOCI [Member]
Balance at Sep. 30, 2018	$ 1,391,622	$ 394	$ 1,412	$ 1,207,644	$ (36,343)	$ 214,569	$ 394	$ 4,340
Net income	24,383					24,383
Other comprehensive income (loss), net of tax	(6,217)							(6,217)
ESOP activity	531			118	413
Stock-based compensation	95			95
Stock options exercised	467		1	466
Cash dividends to stockholders	(65,362)					(65,362)
Balance at Dec. 31, 2018	1,345,913		1,413	1,208,323	(35,930)	173,984		(1,877)
Balance at Sep. 30, 2018	1,391,622	394	1,412	1,207,644	(36,343)	214,569	394	4,340
Net income	71,834
Other comprehensive income (loss), net of tax								(16,872)
Balance at Jun. 30, 2019	1,327,099		1,414	1,209,740	(35,104)	163,581		(12,532)
Balance at Dec. 31, 2018	1,345,913		1,413	1,208,323	(35,930)	173,984		(1,877)
Net income	24,554					24,554
Other comprehensive income (loss), net of tax	(3,539)							(3,539)
ESOP activity	547			134	413
Stock-based compensation	90			90
Stock options exercised	118			118
Cash dividends to stockholders	(11,700)					(11,700)
Balance at Mar. 31, 2019	1,355,983		1,413	1,208,665	(35,517)	186,838		(5,416)
Net income	22,897					22,897
Other comprehensive income (loss), net of tax	(7,116)							(7,116)
ESOP activity	560			147	413
Stock-based compensation	167			167
Stock options exercised	762		1	761
Cash dividends to stockholders	(46,154)					(46,154)
Balance at Jun. 30, 2019	1,327,099		1,414	1,209,740	(35,104)	163,581		(12,532)
Balance at Sep. 30, 2019	1,336,326	88	1,414	1,210,226	(34,692)	174,277	88	(14,899)
Net income	22,511					22,511
Other comprehensive income (loss), net of tax	4,972							4,972
ESOP activity	582			169	413
Restricted stock activity, net	(1)			(1)
Stock-based compensation	166			166
Stock options exercised	613		1	612
Cash dividends to stockholders	(58,663)					(58,663)
Balance at Dec. 31, 2019	1,306,594		1,415	1,211,172	(34,279)	138,213		(9,927)
Balance at Sep. 30, 2019	1,336,326	$ 88	1,414	1,210,226	(34,692)	174,277	$ 88	(14,899)
Net income	46,261
Other comprehensive income (loss), net of tax								(2,692)
Balance at Jun. 30, 2020	1,300,520		1,415	1,211,653	(33,453)	138,496		(17,591)
Balance at Dec. 31, 2019	1,306,594		1,415	1,211,172	(34,279)	138,213		(9,927)
Net income	4,276					4,276
Other comprehensive income (loss), net of tax	(12,051)							(12,051)
ESOP activity	530			117	413
Stock-based compensation	152			152
Stock options exercised	25			25
Cash dividends to stockholders	(11,733)					(11,733)
Balance at Mar. 31, 2020	1,287,793		1,415	1,211,466	(33,866)	130,756		(21,978)
Net income	19,474					19,474
Other comprehensive income (loss), net of tax	4,387							4,387
ESOP activity	474			61	413
Restricted stock activity, net	(6)			(6)
Stock-based compensation	132			132
Cash dividends to stockholders	(11,734)					(11,734)
Balance at Jun. 30, 2020	$ 1,300,520		$ 1,415	$ 1,211,653	$ (33,453)	$ 138,496		$ (17,591)