Investment Securities - Investment Securities With Gross Unrealized Losses (Details) $ in Thousands	Mar. 31, 2021 USD ($) position	Sep. 30, 2020 USD ($) position
Fair Value
Continuous loss position less than twelve months	$ 16,924	$ 2,890
Continuous loss position more than twelve months	2,375	214
Total securities available for sale	$ 19,299	$ 3,104
Number of Investment Positions
Continuous loss position less than twelve months | position	16	3
Continuous loss position more than twelve months | position	4	2
Total securities available for sale | position	20	5
Gross Unrealized Losses
Continuous loss position less than twelve months	$ 440	$ 61
Continuous loss position more than twelve months	172	11
Total securities available for sale	612	72
Agency mortgage-backed
Fair Value
Continuous loss position less than twelve months	$ 3,076
Number of Investment Positions
Continuous loss position less than twelve months | position	1
Gross Unrealized Losses
Continuous loss position less than twelve months	$ 105
Agency CMO
Fair Value
Continuous loss position less than twelve months	$ 1,456
Number of Investment Positions
Continuous loss position less than twelve months | position	2
Gross Unrealized Losses
Continuous loss position less than twelve months	$ 39
Privately-issued CMO
Fair Value
Continuous loss position more than twelve months	$ 23	$ 26
Number of Investment Positions
Continuous loss position more than twelve months | position	1	1
Gross Unrealized Losses
Continuous loss position more than twelve months	$ 7	$ 8
Privately-issued ABS
Fair Value
Continuous loss position less than twelve months		$ 446
Continuous loss position more than twelve months	$ 400
Number of Investment Positions
Continuous loss position less than twelve months | position		1
Continuous loss position more than twelve months | position	1
Gross Unrealized Losses
Continuous loss position less than twelve months		$ 5
Continuous loss position more than twelve months	$ 1
SBA certificates
Fair Value
Continuous loss position less than twelve months	2,179
Continuous loss position more than twelve months	$ 115	$ 188
Number of Investment Positions
Continuous loss position less than twelve months | position	1
Continuous loss position more than twelve months | position	1	1
Gross Unrealized Losses
Continuous loss position less than twelve months	$ 25
Continuous loss position more than twelve months	1	$ 3
Municipal bonds
Fair Value
Continuous loss position less than twelve months	10,213	$ 2,444
Continuous loss position more than twelve months	$ 1,837
Number of Investment Positions
Continuous loss position less than twelve months | position	12	2
Continuous loss position more than twelve months | position	1
Gross Unrealized Losses
Continuous loss position less than twelve months	$ 271	$ 56
Continuous loss position more than twelve months	$ 163