Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Changes in Level 3 securities
Beginning Balance	$ 53,782	$ 18,914
Purchases	6,179	12,730
Maturities and calls	(9,974)	(3,457)
Amortization	(9)	(11)
Changes in fair value
Ending Balance	49,978	28,176
Auction Rate Securities [Member]
Changes in Level 3 securities
Beginning Balance	3,963	3,725
Purchases
Maturities and calls
Amortization
Changes in fair value
Ending Balance	3,963	3,725
Obligations Issued by States & Political Subdivisions [Member]
Changes in Level 3 securities
Beginning Balance	49,477	14,772
Purchases	6,179	12,730
Maturities and calls	(9,974)	(3,434)
Amortization	(9)	(11)
Changes in fair value
Ending Balance	45,673	24,057
Equity Securities [Member]
Changes in Level 3 securities
Beginning Balance	342	417
Purchases
Maturities and calls		(23)
Amortization
Changes in fair value
Ending Balance	$ 342	$ 394

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

No definition available.

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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

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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

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

- Details

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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
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