Note 4 - Allowance for Loan Losses - Segregated by Impairment Methodology (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Ending allowance balance, individually evaluated for impairment	$ 1,487		$ 1,585
Ending allowance balance, collectively evaluated for impairment	5,668		6,392
Ending allowance balance	7,155	$ 7,508	7,977	$ 8,923
Ending loan balance, individually evaluated for impairment	19,557		21,426
Ending loan balance, collectively evaluated for impairment	759,886		748,620
Loans	779,443	765,284	770,046
Ending loan balance	779,443	765,284	770,046
Commercial [Member] \| Commercial and Agricultural Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	0		0
Ending allowance balance, collectively evaluated for impairment	384		436
Ending allowance balance	384	447	436	456
Ending loan balance, individually evaluated for impairment	39		77
Ending loan balance, collectively evaluated for impairment	47,253		45,286
Loans	47,292	48,122	45,363
Ending loan balance	47,292	48,122	45,363
Commercial [Member] \| Real Estate Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	1,394		1,471
Ending allowance balance, collectively evaluated for impairment	2,887		2,752
Ending allowance balance	4,281	3,874	4,223	5,751
Ending loan balance, individually evaluated for impairment	15,164		17,605
Ending loan balance, collectively evaluated for impairment	334,244		328,646
Loans	349,408	351,172	346,251
Ending loan balance	349,408	351,172	346,251
Agricultural [Member] \| Commercial and Agricultural Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	0		0
Ending allowance balance, collectively evaluated for impairment	315		249
Ending allowance balance	315	186	249	168
Ending loan balance, individually evaluated for impairment	27		5
Ending loan balance, collectively evaluated for impairment	21,388		25,241
Loans	21,415	16,443	25,246
Ending loan balance	21,415	16,443	25,246
Commercial Construction [Member] \| Real Estate Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	44		69
Ending allowance balance, collectively evaluated for impairment	81		919
Ending allowance balance	125	1,216	988	323
Ending loan balance, individually evaluated for impairment	471		497
Ending loan balance, collectively evaluated for impairment	53,099		36,036
Loans	53,570	45,214	36,533
Ending loan balance	53,570	45,214	36,533
Residential Construction [Member] \| Real Estate Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	0		0
Ending allowance balance, collectively evaluated for impairment	17		6
Ending allowance balance	17		6	13
Ending loan balance, individually evaluated for impairment	0		0
Ending loan balance, collectively evaluated for impairment	12,897		8,905
Loans	12,897	8,583	8,905
Ending loan balance	12,897	8,583	8,905
Residential [Member] \| Real Estate Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	20		21
Ending allowance balance, collectively evaluated for impairment	1,162		1,108
Ending allowance balance	1,182	968	1,129	1,396
Ending loan balance, individually evaluated for impairment	1,753		2,204
Ending loan balance, collectively evaluated for impairment	187,400		194,128
Loans	189,153	194,049	196,332
Ending loan balance	189,153	194,049	196,332
Farmland [Member] \| Real Estate Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	29		24
Ending allowance balance, collectively evaluated for impairment	701		848
Ending allowance balance	730	780	872	722
Ending loan balance, individually evaluated for impairment	2,103		1,038
Ending loan balance, collectively evaluated for impairment	67,966		70,865
Loans	70,069	67,768	71,903
Ending loan balance	70,069	67,768	71,903
Consumer [Member] \| Consumer and Other Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	0		0
Ending allowance balance, collectively evaluated for impairment	93		56
Ending allowance balance	93	34	56	80
Ending loan balance, individually evaluated for impairment	0		0
Ending loan balance, collectively evaluated for impairment	18,623		18,677
Loans	18,623	18,956	18,677
Ending loan balance	18,623	18,956	18,677
Other [Member] \| Consumer and Other Portfolio Segment [Member]
Ending allowance balance, individually evaluated for impairment	0		0
Ending allowance balance, collectively evaluated for impairment	28		18
Ending allowance balance	28	3	18	$ 14
Ending loan balance, individually evaluated for impairment	0		0
Ending loan balance, collectively evaluated for impairment	17,016		20,836
Loans	17,016	14,977	20,836
Ending loan balance	$ 17,016	$ 14,977	$ 20,836

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

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 (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ 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

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 (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ 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

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 (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ 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=84173941&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=82894903&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=84173941&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=82891408&loc=d3e4428-111522

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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