Stockholders' Equity and Regulatory Capital (Details) $ in Thousands	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)
Southern Bank \| Total Capital (to Risk-Weighted Assets)
Capital	$ 454,699	$ 352,169
Capital to Risk Weighted Assets	0.1177	0.1290
Capital Required for Capital Adequacy	$ 308,932	$ 218,397
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0800	0.0800
Capital Required to be Well Capitalized	$ 386,166	$ 272,996
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Southern Bank \| Tier I Capital (to Risk-Weighted Assets)
Capital	$ 407,764	$ 325,183
Capital to Risk Weighted Assets	0.1056	0.1191
Capital Required for Capital Adequacy	$ 231,699	$ 163,797
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0600	0.0600
Capital Required to be Well Capitalized	$ 308,932	$ 218,397
Capital Required to be Well Capitalized to Risk Weighted Assets	0.0800	0.0800
Southern Bank \| Tier I Capital (to Average Assets)
Capital	$ 407,764	$ 325,183
Capital to Risk Weighted Assets	0.0954	0.1022
Capital Required for Capital Adequacy	$ 170,942	$ 127,333
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0400	0.0400
Capital Required to be Well Capitalized	$ 213,677	$ 159,167
Capital Required to be Well Capitalized to Risk Weighted Assets	0.0500	0.0500
Southern Bank \| Common Equity Tier I Capital (to Risk-Weighted Assets)
Capital	$ 407,764	$ 325,183
Capital to Risk Weighted Assets	0.1056	0.1191
Capital Required for Capital Adequacy	$ 173,774	$ 122,848
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0450	0.0450
Capital Required to be Well Capitalized	$ 251,008	$ 177,447
Capital Required to be Well Capitalized to Risk Weighted Assets	0.0650	0.0650
Consolidated \| Total Capital (to Risk-Weighted Assets)
Capital	$ 481,236	$ 370,013
Capital to Risk Weighted Assets	0.1252	0.1342
Capital Required for Capital Adequacy	$ 307,528	$ 220,558
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0800	0.0800
Consolidated \| Tier I Capital (to Risk-Weighted Assets)
Capital	$ 426,644	$ 335,316
Capital to Risk Weighted Assets	0.1110	0.1216
Capital Required for Capital Adequacy	$ 230,646	$ 165,418
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0600	0.0600
Consolidated \| Tier I Capital (to Average Assets)
Capital	$ 426,644	$ 335,316
Capital to Risk Weighted Assets	0.0995	0.1041
Capital Required for Capital Adequacy	$ 171,470	$ 128,822
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0400	0.0400
Consolidated \| Common Equity Tier I Capital (to Risk-Weighted Assets)
Capital	$ 411,196	$ 319,971
Capital to Risk Weighted Assets	0.1070	0.1161
Capital Required for Capital Adequacy	$ 172,985	$ 124,064
Capital Required for Capital Adequacy to Risk Weighted Assets	0.0450	0.0450

Amount of total capital as defined by regulatory framework.

Name:	us-gaap_Capital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480696/942-505-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480696/942-505-50-1B

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum total risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionAxis=smbc_SouthernBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	smbc_RegulatoryCapitalRequirementsUnderBankingRegulationsAxis=smbc_TotalCapitalToRiskWeightedAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	smbc_RegulatoryCapitalRequirementsUnderBankingRegulationsAxis=smbc_TierICapitalToRiskWeightedAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	smbc_RegulatoryCapitalRequirementsUnderBankingRegulationsAxis=smbc_TierICapitalToAverageAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	smbc_RegulatoryCapitalRequirementsUnderBankingRegulationsAxis=smbc_CommonEquityTierICapitalToRiskWeightedAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=smbc_ConsolidatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: