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, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities
Net income	$ 6,018	$ 7,024	$ 7,251	$ 7,290	$ 6,357	$ 6,211	$ 5,049	$ 6,652	$ 27,583	$ 24,269	$ 24,020
Adjustments to reconcile net income to net cash provided by operating activities:
Earned ESOP shares									0	31	441
Stock option compensation expense									173	182	159
Net cash provided by operating activities									29,635	33,856	18,108
Cash flows from investing activities
Purchase of investment securities held to maturity									(53,049)	(10,255)	(13,166)
Cash acquired, net of cash consideration paid in business combination									0	0	14,284
Net cash provided by (used in) investing activities									37,445	(154,668)	(44,547)
Cash flows from financing activities
Repurchase of common stock									(527)	(1,238)	(499)
Payment of dividends									(8,589)	(7,083)	(6,495)
Net cash provided by financing activities									198,664	292,249	20,590
Cash and cash equivalents:
Beginning of year				314,452				143,015	314,452	143,015	148,864
End of year	580,196				314,452				580,196	314,452	143,015
Parent Company
Cash flows from operating activities
Net income									27,583	24,269	24,020
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of Bank									(18,726)	(15,844)	(17,693)
Earned ESOP shares									0	31	441
Stock option compensation expense									173	182	159
Other, net									(97)	(279)	9
Net cash provided by operating activities									8,933	8,359	6,936
Cash flows from investing activities
Investment in Bank									(149)	(187)	(14,915)
Purchase of investment securities held to maturity									0	(500)	0
Principal repayments on loan receivable from ESOP									0	0	285
Net cash provided by (used in) investing activities									(149)	(687)	(346)
Cash flows from financing activities
Proceeds from exercise of stock options									631	391	401
Repurchase of common stock									(527)	(1,238)	(499)
Payment of dividends									(8,589)	(7,083)	(6,495)
Net cash provided by financing activities									(8,485)	(7,930)	(6,593)
Net increase (decrease) in cash and cash equivalents									299	(258)	(3)
Cash and cash equivalents:
Beginning of year				$ 2,633				$ 2,891	2,633	2,891	2,894
End of year	$ 2,932				$ 2,633				$ 2,932	$ 2,633	$ 2,891