Fair Value Measurements: Fair Value Measurements, Nonrecurring, Valuation Techniques (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Fair Value, Inputs, Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	$ 0	$ 0
Fair Value, Inputs, Level 1 | Other consumer loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 1 | Commercial business loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 1 | Mortgage loans, land
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 1 | Impaired loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 1 | Private label residential
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 1 | OREO and other repossessed assets
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	2
Fair Value, Inputs, Level 2 | Other consumer loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 2 | Commercial business loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 2 | Mortgage loans, land
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 2 | Impaired loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 2 | Private label residential
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring		2
Fair Value, Inputs, Level 2 | OREO and other repossessed assets
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	0	0
Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	1,783	2,211
Fair Value, Inputs, Level 3 | Other consumer loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring		6
Fair Value, Inputs, Level 3 | Commercial business loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	210	408
Fair Value, Inputs, Level 3 | Mortgage loans, land
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	107	114
Fair Value, Inputs, Level 3 | Impaired loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	317	528
Fair Value, Inputs, Level 3 | Impaired loans | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	317	528
Fair Value, Inputs, Level 3 | OREO and other repossessed assets | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	1,466	1,683
Fair Value, Inputs, Level 3 | Private label residential
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring		0
Fair Value, Inputs, Level 3 | OREO and other repossessed assets
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, estimated fair value, nonrecurring	$ 1,466	$ 1,683