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, 2020	Sep. 30, 2019
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, land
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 | Other consumer loans
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
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
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
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
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
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		2
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
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, estimated fair value, nonrecurring	1,783	2,211
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	107	114
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
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	210	408
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	317	528
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
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	$ 1,466	$ 1,683