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Available for Sale Securities - Fair Value of Debt Securities Reported Less Than Their Historical Cost (Details) - USD ($) $ in Millions | Jun. 30, 2025 | Jun. 30, 2024
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Available for Sale Securities |  |
Fair value of certain investments reported less than their historical cost | $ 264.5 | $ 312.9
Certain investments in debt securities reported at less than historical cost, percentage of Company's AFS portfolio | 57.40% | 73.10%

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- Definition Represents the debt securities reported less than their historical cost percent of investment portfolio, as of the indicated date. + References No definition available. + Details
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Namespace Prefix: | smbc_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents the monetary amount of fair value of debt securities reported less than their historical cost, as of the indicated date. + References No definition available. + Details
Name: | smbc_FairValueOfDebtSecuritiesReportedLessThanTheirHistoricalCost
Namespace Prefix: | smbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration