Loans Receivable and Allowance for Loan Losses - Troubled Debt Restructuring (Details) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2018 USD ($) loan	Mar. 31, 2017 USD ($) loan	Mar. 31, 2018 USD ($) loan	Mar. 31, 2017 USD ($) loan
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	2,000	1,000	2,000	4,000
Pre-modification recorded investment	$ 672	$ 83	$ 672	$ 448
Post-modification recorded investment	$ 672	$ 83	$ 672	$ 448
Real estate loans \| One-to-four family
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	1,000	1,000	1,000	3,000
Pre-modification recorded investment	$ 588	$ 83	$ 588	$ 340
Post-modification recorded investment	$ 588	$ 83	$ 588	$ 340
Real estate loans \| Home equity loans and advances
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan			1,000	1,000
Pre-modification recorded investment			$ 84	$ 108
Post-modification recorded investment			$ 84	$ 108
Consumer loans \| Home equity loans and advances
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	1,000	0
Pre-modification recorded investment	$ 84	$ 0
Post-modification recorded investment	$ 84	$ 0

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
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: