Loans and Leases and Allowance for Credit Losses - Recorded investment in loans by portfolio segment and the balance of the allowance for loan losses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	$ 2,512	$ 2,312
Allowance for Credit Losses, Collectively evaluated for impairment	62,672	61,541
Allowance for Credit Losses, PCI	363	760
Allowance for Credit Losses, Ending balance	65,547	64,613	$ 61,226	$ 63,424
Loans Outstanding, Ending Balance	8,812,553	8,383,401
Allowance for unfunded commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	0	0
Allowance for Credit Losses, Collectively evaluated for impairment	3,458	3,410
Allowance for Credit Losses, PCI	0	0
Allowance for Credit Losses, Ending balance	3,458	3,410
Loans Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	2,512	2,312
Allowance for Credit Losses, Collectively evaluated for impairment	59,214	58,131
Allowance for Credit Losses, PCI	363	760
Allowance for Credit Losses, Ending balance	62,089	61,203	58,914	61,422
Loans Outstanding, Individually evaluated for impairment	61,971	55,449
Loans Outstanding, Collectively evaluated for impairment	8,691,936	8,253,503
PCI Loans	58,646	74,449
Loans Outstanding, Ending Balance	8,812,553	8,383,401
Loans Receivable \| Indirect auto
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	0	26
Allowance for Credit Losses, Collectively evaluated for impairment	0	973
Allowance for Credit Losses, PCI	0	0
Allowance for Credit Losses, Ending balance	0	999	1,550	1,802
Loans Outstanding, Individually evaluated for impairment	0	1,237
Loans Outstanding, Collectively evaluated for impairment	0	206,455
PCI Loans	0	0
Loans Outstanding, Ending Balance	0	207,692
Loans Receivable \| Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
PCI Loans	46,947	62,420
Loans Outstanding, Ending Balance	6,669,593	6,099,443
Loans Receivable \| Commercial \| Owner occupied commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	816	862
Allowance for Credit Losses, Collectively evaluated for impairment	10,483	11,328
Allowance for Credit Losses, PCI	105	17
Allowance for Credit Losses, Ending balance	11,404	12,207	14,776	16,446
Loans Outstanding, Individually evaluated for impairment	19,233	17,602
Loans Outstanding, Collectively evaluated for impairment	1,692,448	1,620,450
PCI Loans	8,546	9,852
Loans Outstanding, Ending Balance	1,720,227	1,647,904
Loans Receivable \| Commercial \| Income producing commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	770	402
Allowance for Credit Losses, Collectively evaluated for impairment	11,507	10,671
Allowance for Credit Losses, PCI	29	0
Allowance for Credit Losses, Ending balance	12,306	11,073	9,381	8,843
Loans Outstanding, Individually evaluated for impairment	18,134	16,584
Loans Outstanding, Collectively evaluated for impairment	1,962,588	1,757,525
PCI Loans	27,228	38,311
Loans Outstanding, Ending Balance	2,007,950	1,812,420
Loans Receivable \| Commercial \| Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	21	32
Allowance for Credit Losses, Collectively evaluated for impairment	5,193	4,761
Allowance for Credit Losses, PCI	52	9
Allowance for Credit Losses, Ending balance	5,266	4,802	3,971	3,810
Loans Outstanding, Individually evaluated for impairment	1,449	1,621
Loans Outstanding, Collectively evaluated for impairment	1,218,882	1,276,318
PCI Loans	326	408
Loans Outstanding, Ending Balance	1,220,657	1,278,347
Loans Receivable \| Commercial \| Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	55	71
Allowance for Credit Losses, Collectively evaluated for impairment	9,613	9,974
Allowance for Credit Losses, PCI	0	292
Allowance for Credit Losses, Ending balance	9,668	10,337	10,523	13,405
Loans Outstanding, Individually evaluated for impairment	3,675	2,491
Loans Outstanding, Collectively evaluated for impairment	965,678	787,760
PCI Loans	6,862	5,907
Loans Outstanding, Ending Balance	976,215	796,158
Loans Receivable \| Commercial \| Equipment financing
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	0	0
Allowance for Credit Losses, Collectively evaluated for impairment	7,240	5,045
Allowance for Credit Losses, PCI	144	407
Allowance for Credit Losses, Ending balance	7,384	5,452	0	0
Loans Outstanding, Individually evaluated for impairment	1,027	0
Loans Outstanding, Collectively evaluated for impairment	739,532	556,672
PCI Loans	3,985	7,942
Loans Outstanding, Ending Balance	744,544	564,614
Loans Receivable \| Residential \| Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	47	51
Allowance for Credit Losses, Collectively evaluated for impairment	2,456	2,382
Allowance for Credit Losses, PCI	1	0
Allowance for Credit Losses, Ending balance	2,504	2,433	2,729	3,264
Loans Outstanding, Individually evaluated for impairment	1,256	1,207
Loans Outstanding, Collectively evaluated for impairment	234,807	209,070
PCI Loans	374	734
Loans Outstanding, Ending Balance	236,437	211,011
Loans Receivable \| Residential \| Residential mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	782	861
Allowance for Credit Losses, Collectively evaluated for impairment	7,296	7,410
Allowance for Credit Losses, PCI	3	24
Allowance for Credit Losses, Ending balance	8,081	8,295	10,097	8,545
Loans Outstanding, Individually evaluated for impairment	15,991	14,220
Loans Outstanding, Collectively evaluated for impairment	1,092,046	1,025,862
PCI Loans	9,579	9,150
Loans Outstanding, Ending Balance	1,117,616	1,049,232
Loans Receivable \| Residential \| Home equity lines of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	16	1
Allowance for Credit Losses, Collectively evaluated for impairment	4,541	4,740
Allowance for Credit Losses, PCI	18	11
Allowance for Credit Losses, Ending balance	4,575	4,752	5,177	4,599
Loans Outstanding, Individually evaluated for impairment	992	276
Loans Outstanding, Collectively evaluated for impairment	658,273	692,122
PCI Loans	1,410	1,612
Loans Outstanding, Ending Balance	660,675	694,010
Loans Receivable \| Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Individually evaluated for impairment	5	6
Allowance for Credit Losses, Collectively evaluated for impairment	885	847
Allowance for Credit Losses, PCI	11	0
Allowance for Credit Losses, Ending balance	901	853	710	708
Loans Outstanding, Individually evaluated for impairment	214	211
Loans Outstanding, Collectively evaluated for impairment	127,682	121,269
PCI Loans	336	533
Loans Outstanding, Ending Balance	128,232	122,013
Loans Receivable \| Allowance for unfunded commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for Credit Losses, Ending balance	$ 3,458	$ 3,410	$ 2,312	$ 2,002

Reserve for loans impairments recorded after acquisition as a result of current information and events which result in it being probable that estimated cash flows will be less than previously expected or because actual cash flows are less than cash flows previously expected. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118938799&loc=d3e13503-111538

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

+ Details

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

- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118938799&loc=d3e13503-111538

+ Details

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

- 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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- 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 before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. 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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5074-111524

+ Details

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

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=ucbi_AllowanceForUnfundedLoanCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ucbi_OwnerOccupiedCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ucbi_IncomeProducingCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ucbi_CommercialAndIndustrialClassificationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ucbi_EquipmentFinancingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
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: