Loans and Allowance for Credit Losses - Activity in the Allowance for credit losses (Details) - USD ($) $ in Thousands	3 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Allowance for credit losses:
Balance, beginning of period	$ 51,629	$ 52,516
Provision (benefit) charged to expense	4,141	2,021
Losses charged off	(3,871)	(159)
Recoveries	182	59
Balance, end of period	52,081	54,437
Commercial and industrial
Allowance for credit losses:
Balance, beginning of period	6,952	6,233
Provision (benefit) charged to expense	1,190	(85)
Losses charged off	(341)	(39)
Recoveries	32	5
Balance, end of period	7,833	6,114
Agriculture production
Allowance for credit losses:
Balance, beginning of period	3,374	835
Provision (benefit) charged to expense	221	(51)
Losses charged off	(53)
Recoveries	66
Balance, end of period	3,608	784
Consumer
Allowance for credit losses:
Balance, beginning of period	952	578
Provision (benefit) charged to expense	399	13
Losses charged off	(291)	(72)
Recoveries	84	7
Balance, end of period	1,144	526
All other loans
Allowance for credit losses:
Balance, beginning of period	4	17
Provision (benefit) charged to expense	(2)	(3)
Balance, end of period	2	14
Secured by real estate \| 1-4 Family residential real estate
Allowance for credit losses:
Balance, beginning of period	10,274	10,528
Provision (benefit) charged to expense	1,310	157
Losses charged off	(150)	(48)
Balance, end of period	11,434	10,637
Secured by real estate \| Commercial Real Estate \| Non-owner occupied
Allowance for credit losses:
Balance, beginning of period	12,241	19,055
Provision (benefit) charged to expense	1,148	1,673
Losses charged off	(2,875)
Balance, end of period	10,514	20,728
Secured by real estate \| Commercial Real Estate \| Owner occupied
Allowance for credit losses:
Balance, beginning of period	4,521	4,815
Provision (benefit) charged to expense	(189)	(1)
Balance, end of period	4,332	4,814
Secured by real estate \| Multi-family real estate
Allowance for credit losses:
Balance, beginning of period	4,329	5,447
Provision (benefit) charged to expense	(350)	(376)
Recoveries		47
Balance, end of period	3,979	5,118
Secured by real estate \| Construction and land development
Allowance for credit losses:
Balance, beginning of period	4,788	2,901
Provision (benefit) charged to expense	(154)	774
Losses charged off	(161)
Balance, end of period	4,473	3,675
Secured by real estate \| Agriculture real estate
Allowance for credit losses:
Balance, beginning of period	4,194	2,107
Provision (benefit) charged to expense	568	(80)
Balance, end of period	$ 4,762	$ 2,027

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

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

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

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

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ 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_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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: