Consolidated Statements Of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Unearned Compensation ESOP [Member]	Retained Earnings [Member]	AOCI [Member]
Balance at Sep. 30, 2018	$ 1,391,622	$ 1,412	$ 1,207,644	$ (36,343)	$ 214,569	$ 4,340
Net income	24,383				24,383
Other comprehensive income (loss), net of tax	(6,217)					(6,217)
Cumulative effect of adopting Accounting Standards Update	394				394
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	1,412	1,207,644	(36,343)	214,569	4,340
Net income	48,937
Other comprehensive income (loss), net of tax						(9,756)
Balance at Mar. 31, 2019	1,355,983	1,413	1,208,665	(35,517)	186,838	(5,416)
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)
Balance at Sep. 30, 2019	1,336,326	1,414	1,210,226	(34,692)	174,277	(14,899)
Net income	22,511				22,511
Other comprehensive income (loss), net of tax	4,972					4,972
Cumulative effect of adopting Accounting Standards Update	88				88
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	1,414	1,210,226	(34,692)	174,277	(14,899)
Net income	26,787
Other comprehensive income (loss), net of tax						(7,079)
Cumulative effect of adopting Accounting Standards Update	88
Balance at Mar. 31, 2020	1,287,793	1,415	1,211,466	(33,866)	130,756	(21,978)
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)