Loans and Allowance for Credit Losses - Activity in the Allowance for credit losses (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2024
Allowance for credit losses:
Balance, beginning of period	$ 51,629	$ 52,516	$ 47,820
Provision (benefit) charged to expense	10,971	5,847	6,625
Losses charged off	(10,338)	(6,983)	(2,055)
Recoveries	2,650	249	126
Balance, end of period	54,912	51,629	52,516
Commercial and industrial
Allowance for credit losses:
Balance, beginning of period	6,952	6,233	5,235
Provision (benefit) charged to expense	2,050	2,160	1,356
Losses charged off	(2,467)	(1,508)	(395)
Recoveries	69	67	37
Balance, end of period	6,604	6,952	6,233
Agriculture production
Allowance for credit losses:
Balance, beginning of period	3,374	835	782
Provision (benefit) charged to expense	5,531	3,589	53
Losses charged off	(2,696)	(1,052)
Recoveries	66	2
Balance, end of period	6,275	3,374	835
Consumer
Allowance for credit losses:
Balance, beginning of period	952	578	490
Provision (benefit) charged to expense	805	698	400
Losses charged off	(1,103)	(411)	(350)
Recoveries	389	87	38
Balance, end of period	1,043	952	578
All other loans
Allowance for credit losses:
Balance, beginning of period	4	17	21
Provision (benefit) charged to expense	(3)	(13)	(4)
Recoveries	2
Balance, end of period	3	4	17
Secured by real estate \| 1-4 Family residential real estate
Allowance for credit losses:
Balance, beginning of period	10,274	10,528	9,474
Provision (benefit) charged to expense	2,623	(211)	1,067
Losses charged off	(813)	(89)	(46)
Recoveries	1	46	33
Balance, end of period	12,085	10,274	10,528
Secured by real estate \| Commercial Real Estate \| Non-owner occupied
Allowance for credit losses:
Balance, beginning of period	12,241	19,055	13,863
Provision (benefit) charged to expense	(303)	(3,014)	5,688
Losses charged off	(2,986)	(3,800)	(496)
Recoveries	2,000
Balance, end of period	10,952	12,241	19,055
Secured by real estate \| Commercial Real Estate \| Owner occupied
Allowance for credit losses:
Balance, beginning of period	4,521	4,815	5,168
Provision (benefit) charged to expense	703	(172)	(353)
Losses charged off	(81)	(122)
Recoveries	122
Balance, end of period	5,265	4,521	4,815
Secured by real estate \| Multi-family real estate
Allowance for credit losses:
Balance, beginning of period	4,329	5,447	6,806
Provision (benefit) charged to expense	(1,234)	(1,165)	(880)
Losses charged off			(479)
Recoveries		47
Balance, end of period	3,095	4,329	5,447
Secured by real estate \| Construction and land development
Allowance for credit losses:
Balance, beginning of period	4,788	2,901	3,414
Provision (benefit) charged to expense	(1,395)	1,888	(242)
Losses charged off	(192)	(1)	(289)
Recoveries	1		18
Balance, end of period	3,202	4,788	2,901
Secured by real estate \| Agriculture real estate
Allowance for credit losses:
Balance, beginning of period	4,194	2,107	2,567
Provision (benefit) charged to expense	2,194	2,087	(460)
Balance, end of period	$ 6,388	$ 4,194	$ 2,107

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: