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	Mar. 31, 2024	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	$ 1,847	$ 122
Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	460
Commercial business
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	1,387	122
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 real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0
Fair Value, Inputs, Level 1 | Commercial business
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 real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0
Fair Value, Inputs, Level 2 | Commercial business
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	1,847	122
Fair Value, Inputs, Level 3 | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	460
Fair Value, Inputs, Level 3 | Commercial business
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	$ 1,387	$ 122