Loans and Allowance for Credit Losses - Balance and activity in the Allowance for credit losses (Details) - USD ($) $ in Thousands	3 Months Ended
Sep. 30, 2024	Sep. 30, 2023
Allowance for credit losses:
Balance, beginning of period	$ 52,516	$ 47,820
Provision (benefit) charged to expense	2,021	1,570
Losses charged off	(159)	(335)
Recoveries	59	67
Balance, end of period	54,437	49,122
Commercial and industrial
Allowance for credit losses:
Balance, beginning of period	6,233	5,235
Provision (benefit) charged to expense	(85)	(100)
Losses charged off	(39)	(4)
Recoveries	5	3
Balance, end of period	6,114	5,134
Agriculture production
Allowance for credit losses:
Balance, beginning of period	835	782
Provision (benefit) charged to expense	(51)	(85)
Balance, end of period	784	697
Consumer
Allowance for credit losses:
Balance, beginning of period	578	490
Provision (benefit) charged to expense	13	38
Losses charged off	(72)	(88)
Recoveries	7	13
Balance, end of period	526	453
All other loans
Allowance for credit losses:
Balance, beginning of period	17	21
Provision (benefit) charged to expense	(3)	(5)
Balance, end of period	14	16
Secured by real estate \| 1-4 residential real estate
Allowance for credit losses:
Balance, beginning of period	10,528	9,474
Provision (benefit) charged to expense	157	(101)
Losses charged off	(48)	(35)
Recoveries		33
Balance, end of period	10,637	9,371
Secured by real estate \| Commercial Real Estate \| Non-owner occupied
Allowance for credit losses:
Balance, beginning of period	19,055	13,863
Provision (benefit) charged to expense	1,673	2,641
Balance, end of period	20,728	16,504
Secured by real estate \| Commercial Real Estate \| Owner occupied
Allowance for credit losses:
Balance, beginning of period	4,815	5,168
Provision (benefit) charged to expense	(1)	(289)
Balance, end of period	4,814	4,879
Secured by real estate \| Multi-family real estate
Allowance for credit losses:
Balance, beginning of period	5,447	6,806
Provision (benefit) charged to expense	(376)	(612)
Losses charged off		(97)
Recoveries	47
Balance, end of period	5,118	6,097
Secured by real estate \| Construction and land development
Allowance for credit losses:
Balance, beginning of period	2,901	3,414
Provision (benefit) charged to expense	774	224
Losses charged off		(111)
Recoveries		18
Balance, end of period	3,675	3,545
Secured by real estate \| Agriculture real estate
Allowance for credit losses:
Balance, beginning of period	2,107	2,567
Provision (benefit) charged to expense	(80)	(141)
Balance, end of period	$ 2,027	$ 2,426

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 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

Reference 2: 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

+ 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

+ 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: