Employee Benefits - 401(k) Retirement Plan (Details) - USD ($) $ in Millions	12 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
Employee Benefits
Matching contributions of eligible compensation	4.00%
Additional profit-sharing contributions of eligible salary	5.00%
401(k) Retirement Plan Expense	$ 1.5	$ 1.3	$ 1.3
401(k) Retirement Plan Shares Held	0.389
Vesting period	5 years

Represents the monetary amount of 401(k) Retirement Plan Expense, during the indicated time period.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the 401(k) Retirement Plan Shares Held (number of shares), as of the indicated date.

+ References

No definition available.

+ Details

Name:	smbc_N401KRetirementPlanSharesHeld
Namespace Prefix:	smbc_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration