Investment Securities: Narrative-Realized Gains (Losses) (Details) $ in Thousands	12 Months Ended
Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($) security
Investments [Abstract]
Loss on sale of securities	$ 66	$ 23	$ 80
Held-to-maturity securities, realized loss, number of securities \| security		17	16
Security owned and pledged as collateral	$ 81,028	$ 18,587

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Name:	us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant