The amount of investment paid off during the period in financing receivables modified by troubled debt restructurings.

No definition available.

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

- Definition

Number of financing receivables that have been commitments to borrowers with outstanding troubled debt restructurings.

+ References

No definition available.

+ Details

Name:	dcom_NumberOfLoansCommittedToBorrowers
Namespace Prefix:	dcom_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of loans modified in a manner that met the criteria of a TDR.

+ References

No definition available.

+ Details

Name:	dcom_NumberOfLoansModified
Namespace Prefix:	dcom_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of TDR loans that have defaulted within twelve months following modification.

+ References

No definition available.

+ Details

Name:	dcom_NumberOfTroubledDebtRestructureLoansDefaultedFollowingModificationNextFiscalYear
Namespace Prefix:	dcom_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of troubled debt restructurings loan which were paid off during the period.

+ References

No definition available.

+ Details

Name:	dcom_NumberOfTroubledDebtRestructuringsLoanPaidOff
Namespace Prefix:	dcom_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to the number of troubled debt restructurings that are on a non-accrual status in the period.

+ References

No definition available.

+ Details

Name:	dcom_NumberOfTroubledDebtRestructuringsOnNonAccrualStatus
Namespace Prefix:	dcom_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	dcom_TroubledDebtRestructuringOnReceivablesAbstract
Namespace Prefix:	dcom_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

+ Details

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

- Definition

Amount of financing receivable on nonaccrual status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5111-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919258-210447

+ Details

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

- Details

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

- Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
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LOANS RECEIVABLE AND CREDIT QUALITY, Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) Loan	Sep. 30, 2018 Loan	Sep. 30, 2019 USD ($) Loan	Sep. 30, 2018 Loan	Dec. 31, 2018 USD ($) Loan	Jun. 30, 2019 USD ($)
Troubled Debt Restructuring on Receivables [Abstract]
Number of TDRs paid off	3
TDR loans paid off \| $	$ 4,289
Number of TDRs on non-accrual status			0		0
Amount of TDRs on non-accrual status \| $					$ 0	$ 0
Number of loans modified	0	0	0	0
Number of loan commitments to borrowers with outstanding TDRs	0		0		0
TDRs which defaulted within twelve months following the modification	0	0	0	0
Real Estate Loans [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans			1		4
Balance \| $	$ 9		$ 9		$ 4,369
Real Estate Loans [Member] \| One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans			1		1
Balance \| $	9		$ 9		$ 14
Real Estate Loans [Member] \| Multifamily Residential and Residential Mixed-Use [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans			0		2
Balance \| $	0		$ 0		$ 271
Real Estate Loans [Member] \| Commercial Real Estate and Commercial Mixed-Use [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans			0		1
Balance \| $	$ 0		$ 0		$ 4,084