LOANS AND ALLOWANCE FOR LOAN LOSSES - Schedule of Allowance for Loan Losses Activity (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for loan losses:
Beginning balance	$ 14,651	$ 12,859	$ 11,484
Provision for loan losses	1,250	2,250	2,850
Loans charged-off	(453)	(918)	(2,458)
Recoveries	754	460	983
Ending balance	16,202	14,651	12,859
Allowance ending balance:
Individually evaluated for impairment	1,649	493	134
Collectively evaluated for impairment	14,553	14,158	12,725
Ending balance	16,202	14,651	12,859
Loans:
Individually evaluated for impairment	21,677	11,356	6,340
Collectively evaluated for impairment	1,684,718	1,648,179	1,353,204
Ending balance	1,706,395	1,659,535	1,359,544
Commercial and industrial
Allowance for loan losses:
Beginning balance	1,751	1,581	1,592
Provision for loan losses	(117)	426	272
Loans charged-off	(86)	(367)	(1,080)
Recoveries	508	111	797
Ending balance	2,056	1,751	1,581
Allowance ending balance:
Individually evaluated for impairment		64	17
Collectively evaluated for impairment	2,008	1,687	1,564
Ending balance	2,008	1,751	1,581
Loans:
Individually evaluated for impairment	289	1,022	463
Collectively evaluated for impairment	279,294	260,757	197,045
Ending balance	279,583	261,779	197,508
Construction and development
Allowance for loan losses:
Beginning balance	1,920	1,724	1,161
Provision for loan losses	458	196	563
Ending balance	2,378	1,920	1,724
Allowance ending balance:
Individually evaluated for impairment		4
Collectively evaluated for impairment	2,322	1,916	1,724
Ending balance	2,322	1,920	1,724
Loans:
Individually evaluated for impairment	1,212	1,250
Collectively evaluated for impairment	279,286	236,253	196,774
Ending balance	280,498	237,503	196,774
Commercial real estate
Allowance for loan losses:
Beginning balance	6,025	4,585	3,264
Provision for loan losses	827	1,472	1,405
Loans charged-off		(33)	(84)
Recoveries	1	1
Ending balance	6,853	6,025	4,585
Allowance ending balance:
Individually evaluated for impairment	1,587	341	27
Collectively evaluated for impairment	5,486	5,684	4,558
Ending balance	7,073	6,025	4,585
Loans:
Individually evaluated for impairment	17,483	7,153	4,258
Collectively evaluated for impairment	549,877	575,366	413,879
Ending balance	567,360	582,519	418,137
Farmland
Allowance for loan losses:
Beginning balance	643	523	482
Provision for loan losses	(73)	120	41
Ending balance	570	643	523
Allowance ending balance:
Individually evaluated for impairment	62	78	85
Collectively evaluated for impairment	495	565	438
Ending balance	557	643	523
Loans:
Individually evaluated for impairment	133	140	163
Collectively evaluated for impairment	57,343	67,705	58,860
Ending balance	57,476	67,845	59,023
1-4 family residential
Allowance for loan losses:
Beginning balance	2,868	3,022	3,960
Provision for loan losses	268	(196)	(418)
Loans charged-off	(14)	(93)	(543)
Recoveries	3	135	23
Ending balance	3,125	2,868	3,022
Allowance ending balance:
Individually evaluated for impairment		6	5
Collectively evaluated for impairment	3,050	2,862	3,017
Ending balance	3,050	2,868	3,022
Loans:
Individually evaluated for impairment	2,498	1,383	842
Collectively evaluated for impairment	409,668	391,684	373,529
Ending balance	412,166	393,067	374,371
Multi-family residential
Allowance for loan losses:
Beginning balance	631	629	281
Provision for loan losses	(222)	2	348
Ending balance	409	631	629
Allowance ending balance:
Collectively evaluated for impairment	400	631	629
Ending balance	400	631	629
Loans:
Individually evaluated for impairment			217
Collectively evaluated for impairment	37,379	38,386	36,357
Ending balance	37,379	38,386	36,574
Consumer
Allowance for loan losses:
Beginning balance	565	602	585
Provision for loan losses	(2)	127	253
Loans charged-off	(72)	(254)	(344)
Recoveries	111	90	108
Ending balance	602	565	602
Allowance ending balance:
Collectively evaluated for impairment	588	565	602
Ending balance	588	565	602
Loans:
Collectively evaluated for impairment	53,245	54,777	51,267
Ending balance	53,245	54,777	51,267
Agricultural
Allowance for loan losses:
Beginning balance	238	187	153
Provision for loan losses	(41)	(12)	276
Loans charged-off	(89)	(2)	(242)
Recoveries	89	65
Ending balance	197	238	187
Allowance ending balance:
Collectively evaluated for impairment	192	238	187
Ending balance	192	238	187
Loans:
Individually evaluated for impairment	62	408	397
Collectively evaluated for impairment	18,297	22,869	25,199
Ending balance	18,359	23,277	25,596
Overdrafts
Allowance for loan losses:
Beginning balance	10	6	6
Provision for loan losses	152	115	110
Loans charged-off	(192)	(169)	(165)
Recoveries	42	58	55
Ending balance	12	10	6
Allowance ending balance:
Collectively evaluated for impairment	12	10	6
Ending balance	12	10	6
Loans:
Collectively evaluated for impairment	329	382	294
Ending balance	$ 329	$ 382	$ 294

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossAdditionalInformationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118955202&loc=SL82895884-210446

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetReportedAmountAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnty_ConstructionAndDevelopmentPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnty_FarmlandPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnty_OneToFourFamilyResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnty_MultiFamilyResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnty_AgriculturalPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnty_OverdraftsPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: