Fair Value Measurements: Balances of assets measured at estimated fair value on a non-recurring basis, and total losses resulting from estimated fair value adjustments (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2023	Sep. 30, 2022
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	$ 0	$ 0
Fair Value, Inputs, Level 1 | Commercial business loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 2 | Commercial business loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	122	123
Fair Value, Inputs, Level 3 | Commercial business loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	$ 122	$ 123