Fair Value Measurements: Balances of Assets and Liabilities Measured on Nonrecurring Basis (Details) - Nonrecurring - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
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 | Mortgage loans, one-to-four family
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 1 | Mortgage loans, land
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 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 1 | Total impaired 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 1 | Private label residential
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 | OREO and other repossessed items
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	10	8
Fair Value, Inputs, Level 2 | Mortgage loans, one-to-four family
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 2 | Mortgage loans, land
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 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 | Total impaired 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 | Private label residential
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	10	8
Fair Value, Inputs, Level 2 | OREO and other repossessed items
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	566	1,741
Fair Value, Inputs, Level 3 | Mortgage loans, one-to-four family
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring		481
Fair Value, Inputs, Level 3 | Mortgage loans, land
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	286
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	123	210
Fair Value, Inputs, Level 3 | Total impaired loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	409	691
Fair Value, Inputs, Level 3 | Private label residential
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 | OREO and other repossessed items
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	$ 157	$ 1,050