Employee Stock Ownership and 401(k) Plan: Narrative (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 1998 USD ($) shares	Sep. 30, 2021 USD ($) component shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
ESOP shares (in shares) | shares		397,626	415,698	425,281
Earned ESOP shares		$ 0	$ 31,000	$ 441,000
401(k)
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
401(k), employer safe harbor contribution (as a percent)		3.00%
401(k), employer contributions		$ 931,000	908,000	743,000
KSOP
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Number of components in KSOP | component		2
Required service period (in years)		1 year
Employee minimum age requirement (in years)		18 years
Award vesting period (in years)		6 years
ESOP
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
ESOP, loan borrowed from employer	$ 7,930,000
ESOP, shares purchased for plan (in shares) | shares	1,058,000
Dividends used for debt service				176,000
ESOP, shares distributed to participants (in shares) | shares		660,374
Earned ESOP shares		$ 0	$ 0	$ 318,000