Loans and Allowance for Credit Losses - Classes of loans (Details) $ in Thousands	3 Months Ended	12 Months Ended
Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate Loans	$ 3,139,090	$ 2,843,499
Loans in Process	(162,101)	(123,656)
Deferred loan fees, net	(380)	(453)
Allowance for credit losses	(37,418)	(33,192)	$ (32,543)	$ (33,222)
Total loans	$ 2,939,191	$ 2,686,198
Number of purchased participation loans	38	31
Purchased participation loans	$ 77,200	$ 70,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate Loans	1,006,925	904,160
Allowance for credit losses	(11,937)	(8,908)	(10,634)	(11,192)
Construction Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate Loans	316,930	258,072
Allowance for credit losses	(2,503)	(2,220)	(2,045)	(2,170)
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate Loans	1,232,331	1,146,673
Allowance for credit losses	(17,886)	(16,838)	(14,883)	(14,535)
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate Loans	100,170	92,996
Allowance for credit losses	(693)	(710)	(873)	(916)
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate Loans	482,734	441,598
Allowance for credit losses	$ (4,399)	$ (4,516)	$ (4,108)	$ (4,409)

Represents the pure numeric value of Number of Purchased Participation Loans, during the indicated time period.

No definition available.

Name:	smbc_NumberOfPurchasedParticipationLoans
Namespace Prefix:	smbc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the monetary amount of Purchased Participation Loans, as of the indicated date.

+ References

No definition available.

+ Details

Name:	smbc_PurchasedParticipationLoans
Namespace Prefix:	smbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Unearned discounts (other than cash or quantity discounts and the like), finance charges, and interest included in the face amount of receivables, that are shown as a deduction from the related receivables. For example, 1) finance charges booked as a receivable when a loan is made and recognized as income at a later date; and 2) interest charges deducted from the face loan amount, resulting in a discounted amount actually advanced to the borrower (wherein the receivable includes the amount actually advanced to the borrower and the as yet unearned interest income).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4519-111522

+ Details

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

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

- Definition

Amount excluding accrued interest, after allowance for credit loss, of financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableBeforeFeesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of undisbursed portion of the loan receivable balance (loans in process). Excludes loans and leases covered under loss sharing agreements.

+ References

No definition available.

+ Details

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

- Details

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