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, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net income	$ 6,357	$ 6,211	$ 5,049	$ 6,652	$ 6,335	$ 5,956	$ 6,114	$ 5,615	$ 24,269	$ 24,020	$ 16,721
Adjustments to reconcile net income to net cash provided by operating activities:
Earned ESOP shares									31	441	882
Stock option compensation expense									182	159	172
Net cash provided by operating activities									33,856	18,108	21,450
Cash flows from investing activities
Purchase of investment securities held to maturity									(10,255)	(13,166)	(6,073)
Cash acquired, net of cash consideration paid in business combination									0	14,284	0
Net cash used in investing activities									(154,668)	(44,547)	(68,269)
Cash flows from financing activities
Repurchase of common stock									(1,238)	(499)	0
Payment of dividends									(7,083)	(6,495)	(4,431)
Net cash provided by financing activities									292,249	20,590	47,495
Cash and cash equivalents:
Beginning of year				143,015				148,864	143,015	148,864	148,188
End of year	314,452				143,015				314,452	143,015	148,864
Parent Company
Cash flows from operating activities
Net income									24,269	24,020	16,721
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of Bank									(15,844)	(17,693)	(12,595)
Earned ESOP shares									31	441	882
Stock option compensation expense									182	159	172
Other, net									(279)	9	280
Net cash provided by operating activities									8,359	6,936	5,460
Cash flows from investing activities
Investment in Bank									(187)	(14,915)	(1,271)
Purchase of investment securities held to maturity									(500)	0	0
Principal repayments on loan receivable from ESOP									0	285	536
Net cash used in investing activities									(687)	(346)	(735)
Cash flows from financing activities
Proceeds from exercise of stock options									391	401	318
Repurchase of common stock									(1,238)	(499)	0
Payment of dividends									(7,083)	(6,495)	(4,431)
Net cash provided by financing activities									(7,930)	(6,593)	(4,113)
Net increase (decrease) in cash and cash equivalents									(258)	(3)	612
Cash and cash equivalents:
Beginning of year				$ 2,891				$ 2,894	2,891	2,894	2,282
End of year	$ 2,633				$ 2,891				$ 2,633	$ 2,891	$ 2,894