Condensed Financial Information - Parent Company Only: Condensed Statements of Cash Flows, Parent Company (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Cash flows from operating activities
Net income	$ 6,642	$ 6,306	$ 6,663	$ 7,507	$ 7,050	$ 5,738	$ 5,327	$ 5,485	$ 27,118	$ 23,600	$ 27,583
Adjustments to reconcile net income to net cash provided by operating activities:
Stock option compensation expense									320	246	173
Net cash provided by operating activities									30,997	26,500	29,635
Cash flows from investing activities
Purchase of investment securities held to maturity									(15,602)	(208,778)	(53,049)
Net cash provided by (used in) investing activities									(170,223)	(335,162)	37,445
Cash flows from financing activities
Proceeds from exercise of stock options									698	415	631
Repurchase of common stock									(4,998)	(4,583)	(527)
Payment of dividends									(8,267)	(7,232)	(8,589)
Net cash (used in) provided by financing activities									(48,808)	45,221	198,664
Net increase (decrease) in cash and cash equivalents									(188,034)	(263,441)	265,744
Cash and cash equivalents:
Beginning of year				316,755					316,755
End of year	128,721				316,755				128,721	316,755
Parent Company
Cash flows from operating activities
Net income									27,118	23,600	27,583
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of Bank									(15,897)	(13,482)	(18,726)
Stock option compensation expense									320	246	173
Other, net									100	16	(97)
Net cash provided by operating activities									11,641	10,380	8,933
Cash flows from investing activities
Investment in Bank									(267)	(202)	(149)
Net cash provided by (used in) investing activities									(267)	(202)	(149)
Cash flows from financing activities
Proceeds from exercise of stock options									698	415	631
Repurchase of common stock									(4,998)	(4,583)	(527)
Payment of dividends									(8,267)	(7,232)	(8,589)
Net cash (used in) provided by financing activities									(12,567)	(11,400)	(8,485)
Net increase (decrease) in cash and cash equivalents									(1,193)	(1,222)	299
Cash and cash equivalents:
Beginning of year				$ 1,710				$ 2,932	1,710	2,932	2,633
End of year	$ 517				$ 1,710				$ 517	$ 1,710	$ 2,932