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) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Fair Value, Inputs, Level 1 | Consumer loans, home equity and second mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring		$ 0
Fair Value, Inputs, Level 2 | Consumer loans, home equity and second mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 3 | Consumer loans, home equity and second mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring		0
Nonrecurring | 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
Nonrecurring | 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	0
Nonrecurring | Fair Value, Inputs, Level 1 | Other consumer loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Nonrecurring | 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
Nonrecurring | 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
Nonrecurring | 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
Nonrecurring | Fair Value, Inputs, Level 1 | OREO and other repossessed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Nonrecurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	2
Nonrecurring | 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	0
Nonrecurring | Fair Value, Inputs, Level 2 | Other consumer loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Nonrecurring | 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
Nonrecurring | 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
Nonrecurring | 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		2
Nonrecurring | Fair Value, Inputs, Level 2 | OREO and other repossessed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	0	0
Nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	2,123	2,211
Nonrecurring | 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	111	114
Nonrecurring | Fair Value, Inputs, Level 3 | Other consumer loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	6	6
Nonrecurring | 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	347	408
Nonrecurring | 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	464	528
Nonrecurring | 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
Nonrecurring | Fair Value, Inputs, Level 3 | OREO and other repossessed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	$ 1,659	$ 1,683