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Loans and Allowance for Credit Losses - PCD loans credit quality indicators (Details) $ in Thousands | Dec. 31, 2020 USD ($)
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Pass |
Purchased Credit Deteriorated Loans | $ 3,200
Watch |
Purchased Credit Deteriorated Loans | 7,800
Special Mention |
Purchased Credit Deteriorated Loans | 0
Substandard |
Purchased Credit Deteriorated Loans | 5,200
Doubtful |
Purchased Credit Deteriorated Loans | $ 0

X
- Definition Represents the monetary amount of Purchased Credit Deteriorated Loans, as of the indicated date. + References No definition available. + Details
Name: | smbc_PurchasedCreditDeterioratedLoans
Namespace Prefix: | smbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=smbc_WatchMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |