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401(k) Retirement Plan (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Retirement plan expenses	$ 413	$ 343	$ 869	$ 724
Vesting period			5 years
Maximum
Defined Contribution Plan Disclosure [Line Items]
Matching contribution (in percent)			4.00%

X

- Definition

Vesting term of retirement plans.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of administrative expense incurred by defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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

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

X

- Definition

Percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: