Loans and Allowance for Credit Losses - Allowance for off-balance credit exposure (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2023
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	$ 3,263	$ 6,288	$ 3,358
Provision (benefit) charged to expense	676	(3,025)	2,930
Balance, end of period	3,939	3,263	6,288
Commercial and industrial
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	782	730	420
Provision (benefit) charged to expense	292	52	310
Balance, end of period	1,074	782	730
Agriculture production
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	37	107	218
Provision (benefit) charged to expense	(37)	(70)	(111)
Balance, end of period		37	107
Consumer
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	12	16	2
Provision (benefit) charged to expense	(12)	(4)	14
Balance, end of period		12	16
All other loans
Allowance for off-balance sheet credit exposure:
Balance, beginning of period		10
Provision (benefit) charged to expense	8	(10)	10
Balance, end of period	8		10
Secured by real estate \| 1-4 residential real estate
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	140	126	107
Provision (benefit) charged to expense	62	14	19
Balance, end of period	202	140	126
Secured by real estate \| Commercial Real Estate \| Non-owner occupied
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	153	154	202
Provision (benefit) charged to expense	(19)	(1)	(48)
Balance, end of period	134	153	154
Secured by real estate \| Commercial Real Estate \| Owner occupied
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	136	182	78
Provision (benefit) charged to expense	25	(46)	104
Balance, end of period	161	136	182
Secured by real estate \| Multi-family real estate
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	31	16	15
Provision (benefit) charged to expense	(31)	15	1
Balance, end of period		31	16
Secured by real estate \| Construction and land development
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	1,912	4,897	2,243
Provision (benefit) charged to expense	367	(2,985)	2,654
Balance, end of period	2,279	1,912	4,897
Secured by real estate \| Agriculture real estate
Allowance for off-balance sheet credit exposure:
Balance, beginning of period	60	50	73
Provision (benefit) charged to expense	21	10	(23)
Balance, end of period	$ 81	$ 60	$ 50

Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482454/460-10-45-1

+ Details

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

- Definition

Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476148/220-40-50-22

+ Details

Name:	us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_OffBalanceSheetCreditLossLiabilityRollforward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=smbc_CommercialAndIndustrialPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=smbc_AgricultureProductionPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=srt_OtherPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_RealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=srt_SingleFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_CommercialBorrowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_ConsumerBorrowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=smbc_ConstructionPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=smbc_AgricultureRealEstatePortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: