Loans and Allowance for Credit Losses - Activity in the Allowance for credit losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Allowance for credit losses:
Balance, beginning of period	$ 54,465	$ 54,740	$ 51,629	$ 52,516
Provision (benefit) charged to expense	1,846	1,300	7,667	3,822
Losses charged off	(503)	(1,119)	(5,882)	(1,558)
Recoveries	129	19	2,523	160
Balance, end of period	55,937	54,940	55,937	54,940
Commercial and industrial
Allowance for credit losses:
Balance, beginning of period	8,804	8,063	6,952	6,233
Provision (benefit) charged to expense	(420)	(415)	2,258	1,443
Losses charged off	(116)	(88)	(980)	(153)
Recoveries	25	12	63	49
Balance, end of period	8,293	7,572	8,293	7,572
Agriculture production
Allowance for credit losses:
Balance, beginning of period	2,379	1,060	3,374	835
Provision (benefit) charged to expense	1,447	1,105	502	1,330
Losses charged off		(976)	(116)	(976)
Recoveries		2	66	2
Balance, end of period	3,826	1,191	3,826	1,191
Consumer
Allowance for credit losses:
Balance, beginning of period	1,343	603	952	578
Provision (benefit) charged to expense	(50)	533	696	748
Losses charged off	(340)	(45)	(862)	(246)
Recoveries	103	5	270	16
Balance, end of period	1,056	1,096	1,056	1,096
All other loans
Allowance for credit losses:
Balance, beginning of period	3	24	4	17
Provision (benefit) charged to expense	1	(7)
Balance, end of period	4	17	4	17
Secured by real estate \| 1-4 Family residential real estate
Allowance for credit losses:
Balance, beginning of period	10,735	12,664	10,274	10,528
Provision (benefit) charged to expense	188	(1,323)	1,455	817
Losses charged off		(10)	(807)	(60)
Recoveries			1	46
Balance, end of period	10,923	11,331	10,923	11,331
Secured by real estate \| Commercial Real Estate \| Non-owner occupied
Allowance for credit losses:
Balance, beginning of period	11,622	13,660	12,241	19,055
Provision (benefit) charged to expense	(101)	3,912	155	(1,483)
Losses charged off			(2,875)
Recoveries			2,000
Balance, end of period	11,521	17,572	11,521	17,572
Secured by real estate \| Commercial Real Estate \| Owner occupied
Allowance for credit losses:
Balance, beginning of period	5,125	5,707	4,521	4,815
Provision (benefit) charged to expense	80	(627)	596	387
Losses charged off	(47)		(81)	(122)
Recoveries			122
Balance, end of period	5,158	5,080	5,158	5,080
Secured by real estate \| Multi-family real estate
Allowance for credit losses:
Balance, beginning of period	3,883	5,725	4,329	5,447
Provision (benefit) charged to expense	(175)	(485)	(621)	(254)
Recoveries				47
Balance, end of period	3,708	5,240	3,708	5,240
Secured by real estate \| Construction and land development
Allowance for credit losses:
Balance, beginning of period	5,745	4,717	4,788	2,901
Provision (benefit) charged to expense	(265)	(1,330)	853	487
Losses charged off			(161)	(1)
Recoveries	1		1
Balance, end of period	5,481	3,387	5,481	3,387
Secured by real estate \| Agriculture real estate
Allowance for credit losses:
Balance, beginning of period	4,826	2,517	4,194	2,107
Provision (benefit) charged to expense	1,141	(63)	1,773	347
Balance, end of period	$ 5,967	$ 2,454	$ 5,967	$ 2,454

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: