LOAN SECURITIZATION (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Percentage of original principal amount, maximum	10.00%
Reimbursement agreement liability with FHLMC	$ 420	$ 420
Multifamily Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loan securitization amount		$ 280,186

Refers to maximum original amount percentage by the entity is obligated to reimburse to FHLMC.

No definition available.

Name:	dcom_PercentageOfOriginalPrincipalAmountMaximum
Namespace Prefix:	dcom_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Repayment paid or payable for expenses incurred on behalf of a client or customer.

+ References

No definition available.

+ Details

Name:	dcom_ReimbursementAgreementLiability
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Definition

This is the principal amount outstanding for securitized loans only (across all types of loans).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

+ Details

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

- Details

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