Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Changes in Level 3 securities
Beginning Balance	$ 18,914	$ 20,660
Purchases	90,960	25,459
Maturities	(56,289)	(26,535)
Amortization	(41)	(2)
Changes in fair value	238	(668)
Ending Balance	53,782	18,914
Auction Rate Securities [Member]
Changes in Level 3 securities
Beginning Balance	3,725	4,393
Purchases
Maturities
Amortization
Changes in fair value	238	(668)
Ending Balance	3,963	3,725
Obligations Issued by States & Political Subdivisions [Member]
Changes in Level 3 securities
Beginning Balance	14,772	15,988
Purchases	90,960	25,314
Maturities	(56,214)	(26,528)
Amortization	(41)	(2)
Changes in fair value
Ending Balance	49,477	14,772
Equity Securities [Member]
Changes in Level 3 securities
Beginning Balance	417	279
Purchases		145
Maturities	(75)	(7)
Amortization
Changes in fair value
Ending Balance	$ 342	$ 417

Fair value measurement with unobservable inputs reconciliation recurring basis asset amortization.

No definition available.

Name:	cnbka_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAmortization
Namespace Prefix:	cnbka_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset change in fair value.

+ References

No definition available.

+ Details

Name:	cnbka_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetChangeInFairValue
Namespace Prefix:	cnbka_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset maturities.

+ References

No definition available.

+ Details

Name:	cnbka_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetMaturities
Namespace Prefix:	cnbka_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Definition

Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

+ Details

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

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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