LOANS AND ALLOWANCE FOR CREDIT LOSSES - Narrative (Details)	6 Months Ended	12 Months Ended
Jun. 30, 2020 USD ($) contract	Jun. 30, 2019 contract	Dec. 31, 2019 USD ($) contract
Receivables [Abstract]
Loans classified in doubtful or loss risk rating	$ 0		$ 0
Number of loans modified as TDRs \| contract	0	4	4
Increase in allowance for loan losses due to TDRs			$ 0
Troubled debt restructuring charge-offs			$ 0

Financing receivables, impaired, troubled debt restructuring, increase in allowance for loan losses.

No definition available.

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Namespace Prefix:	gnty_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Loans classified in doubtful and loss risk rating.

+ References

No definition available.

+ Details

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Namespace Prefix:	gnty_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

+ Details

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

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

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

- References

No definition available.

+ Details

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Period Type:	duration