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LOANS AND ALLOWANCE FOR CREDIT LOSSES - Summary of Loan Portfolio by Type of Loan (Details) (Parenthetical) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
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Receivables [Abstract] |  |
Accrued interest receivable on loans | $ 6.7 | $ 6.4

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- Definition Financing receivable accrued interest. + References No definition available. + Details
Name: | gnty_FinancingReceivableAccruedInterest
Namespace Prefix: | gnty_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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