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Commitments and Contingencies (FY) (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Commitments and Contingencies Disclosure [Abstract] |  |
Commitments to extend credit | $ 299.6 | $ 292.8
Standby letters of credit, issued by the Company | $ 3.7 | $ 5.5

X
- Definition Contractual amount for all off-balance sheet commitments. + References No definition available. + Details
Name: | smbk_OutstandingOffBalanceSheetRisksAmountLiability
Namespace Prefix: | smbk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Unused amount of commitments to extend credit to customers. + References No definition available. + Details
Name: | us-gaap_UnusedCommitmentsToExtendCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant