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Organization and Summary of Significant Accounting Policies - Wealth Management Assets and Fees (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025
Organization and Summary of Significant Accounting Policies
Fiduciary assets	$ 108.0	$ 107.6
Investment management assets	$ 562.6	$ 538.2

X

- Definition

The carrying amount as of the balance sheet date of fiduciary assets.

+ References

No definition available.

+ Details

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Namespace Prefix:	smbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

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Namespace Prefix:	smbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The carrying amount of assets an investment adviser manages on behalf of investors.

+ References

No definition available.

+ Details

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