Long-term Debt - Schedule of long-term debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Less discount	$ (8,588)	$ (9,309)
Long-term debt	212,664	267,189
Securitized notes payable
Debt Instrument [Line Items]
Long-term debt, gross	0	54,996
Securitized notes payable \| NER 16-1 Class A-2 notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	19,975
Issue Date	2016
Stated Maturity Date	2021
Interest Rate (percent)	2.20%
Securitized notes payable \| NER 16-1 Class B notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	25,489
Issue Date	2016
Stated Maturity Date	2021
Interest Rate (percent)	3.22%
Securitized notes payable \| NER 16-1 Class C notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	6,319
Issue Date	2016
Stated Maturity Date	2021
Interest Rate (percent)	5.05%
Securitized notes payable \| NER 16-1 Class D notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	3,213
Issue Date	2016
Stated Maturity Date	2023
Interest Rate (percent)	7.87%
Senior debentures
Debt Instrument [Line Items]
Long-term debt, gross	$ 85,000	85,000
Senior debentures \| 2022 senior debentures
Debt Instrument [Line Items]
Long-term debt, gross	$ 50,000	50,000
Issue Date	2015
Stated Maturity Date	2022
Earliest Call Date	2020
Interest Rate (percent)	5.00%
Variable rate basis	3-month LIBOR
Senior debentures \| 2022 senior debentures \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	3.814%
Senior debentures \| 2027 senior debentures
Debt Instrument [Line Items]
Long-term debt, gross	$ 35,000	35,000
Issue Date	2015
Stated Maturity Date	2027
Earliest Call Date	2025
Interest Rate (percent)	5.50%
Variable rate basis	3-month LIBOR
Senior debentures \| 2027 senior debentures \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	3.71%
Subordinated debentures
Debt Instrument [Line Items]
Long-term debt, gross	$ 111,250	111,500
Subordinated debentures \| 2028 subordinated debentures
Debt Instrument [Line Items]
Long-term debt, gross	$ 100,000	100,000
Issue Date	2018
Stated Maturity Date	2028
Earliest Call Date	2023
Interest Rate (percent)	4.50%
Variable rate basis	3-month LIBOR
Subordinated debentures \| 2028 subordinated debentures \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	2.12%
Subordinated debentures \| 2025 subordinated debentures
Debt Instrument [Line Items]
Long-term debt, gross	$ 11,250	11,500
Issue Date	2015
Stated Maturity Date	2025
Earliest Call Date	2020
Interest Rate (percent)	6.25%
Trust preferred securities
Debt Instrument [Line Items]
Long-term debt, gross	$ 25,002	25,002
Trust preferred securities \| Southern Bancorp Capital Trust I
Debt Instrument [Line Items]
Long-term debt, gross	$ 4,382	4,382
Issue Date	2004
Stated Maturity Date	2034
Earliest Call Date	2009
Variable rate basis	Prime
Trust preferred securities \| Southern Bancorp Capital Trust I \| Prime Rate
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.00%
Trust preferred securities \| Tidelands Statutory Trust I
Debt Instrument [Line Items]
Long-term debt, gross	$ 8,248	8,248
Issue Date	2006
Stated Maturity Date	2036
Earliest Call Date	2011
Variable rate basis	3-month LIBOR
Trust preferred securities \| Tidelands Statutory Trust I \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.38%
Trust preferred securities \| Four Oaks Statutory Trust I
Debt Instrument [Line Items]
Long-term debt, gross	$ 12,372	$ 12,372
Issue Date	2006
Stated Maturity Date	2036
Earliest Call Date	2011
Variable rate basis	3-month LIBOR
Trust preferred securities \| Four Oaks Statutory Trust I \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.35%

This element represents debt instrument earliest call year.

No definition available.

Name:	ucbi_DebtInstrumentEarliestCallYear
Namespace Prefix:	ucbi_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents debt instrument issue year.

+ References

No definition available.

+ Details

Name:	ucbi_DebtInstrumentIssueYear
Namespace Prefix:	ucbi_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents debt instruments maturity year.

+ References

No definition available.

+ Details

Name:	ucbi_DebtInstrumentsMaturityYear
Namespace Prefix:	ucbi_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: 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(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: 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(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_NavitasEquipmentReceivablesLlc2016-1ClassA2NotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_NavitasEquipmentReceivablesLlc2016-1ClassBNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_NavitasEquipmentReceivablesLlc2016-1ClassCNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_NavitasEquipmentReceivablesLlc2016-1ClassDNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_TwoThousandTwentyTwoSeniorDebenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_TwoThousandTwentySevenSeniorDebenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_TwoThousandTwentyEightSubordinatedDebenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_TwoThousandTwentyFiveSubordinatedDebenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=ucbi_TrustPreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_SouthernBancorpCapitalTrustOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_TidelandsStatutoryTrustOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ucbi_FourOaksStatutoryTrustIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: