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Premises and Equipment - Calculated amount of right of use assets and lease liabilities (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2021
Right of use assets obtained in exchange for lease obligations: Operating Leases | $ 35 | $ 22 |
Consolidated Balance Sheet [Member] |  |  |
Operating leases right of use asset | 2,735 |  | $ 2,770
Operating leases liability | 2,735 |  | $ 2,770
Consolidated Statement Of Income [Member] |  |  |
Operating lease costs classified as occupancy and equipment expense (includes short-term lease costs) | 100 | 72 |
Supplemental Disclosures Of Cash Flow Information [Member] | Cash paid for amounts included in the measurement of lease liabilities |  |  |
Operating cash flows from operating leases | $ 85 | $ 67 |

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- Definition Represents the monetary amount of Liability, Operating Leases, during the indicated time period. + References No definition available. + Details
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Namespace Prefix: | smbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the monetary amount of Operating Cash Flows from Operating Leases, during the indicated time period. + References No definition available. + Details
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Namespace Prefix: | smbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the monetary amount of Operating Lease Costs Classified as Occupancy and Equipment Expense, during the indicated time period. + References No definition available. + Details
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Namespace Prefix: | smbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the monetary amount of Right of Use Asset, Operating Leases, as of the indicated date. + References No definition available. + Details
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Namespace Prefix: | smbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of increase in right-of-use asset obtained in exchange for operating lease liability. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 55 -Paragraph 53 -URI http://asc.fasb.org/extlink&oid=121568110&loc=SL77918982-209971 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (g)(2) -URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918686-209980 + Details
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Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
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Balance Type: |
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- Details
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Namespace Prefix: |
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Balance Type: |
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- Details
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- Details
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