Investment Securities: Schedule of significant inputs utilized to measure management's estimate of the credit loss component on OTTI securities (Details)	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Minimum
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
OTTI significant inputs - Constant prepayment rate	6.00%	6.00%	6.00%
OTTI significant inputs - Collateral default rate	0.00%	0.58%	1.47%
OTTI significant inputs - Loss severity rate	0.00%	0.00%	0.00%
Maximum
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
OTTI significant inputs - Constant prepayment rate	15.00%	15.00%	15.00%
OTTI significant inputs - Collateral default rate	26.71%	25.64%	17.55%
OTTI significant inputs - Loss severity rate	5.73%	8.19%	12.96%
Weighted Average
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
OTTI significant inputs - Constant prepayment rate	8.26%	12.98%	10.20%
OTTI significant inputs - Collateral default rate	11.28%	9.96%	12.19%
OTTI significant inputs - Loss severity rate	1.55%	3.36%	4.55%