FAIR VALUE OF FINANCIAL INSTRUMENTS, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Recurring [Member]
Trading securities (Registered Mutual Funds) [Abstract]
Domestic Equity Mutual Funds	$ 460	$ 873
International Equity Mutual Funds	120	213
Fixed Income Mutual Funds	2,135	5,867
Registered Mutual Funds [Abstract]
Domestic Equity Mutual Funds	1,512	1,356
International Equity Mutual Funds	445	377
Fixed Income Mutual Funds	2,049	2,162
Pass-through MBS issued by GSEs	340,879	372
Agency CMOs	10,505	3,186
Derivative - interest rate product	4,041	3,228
Financial Liabilities [Abstract]
Liabilities measured at fair value	0	0
Recurring [Member] \| Level 1 Inputs [Member]
Trading securities (Registered Mutual Funds) [Abstract]
Domestic Equity Mutual Funds	460	873
International Equity Mutual Funds	120	213
Fixed Income Mutual Funds	2,135	5,867
Registered Mutual Funds [Abstract]
Domestic Equity Mutual Funds	1,512	1,356
International Equity Mutual Funds	445	377
Fixed Income Mutual Funds	2,049	2,162
Pass-through MBS issued by GSEs	0	0
Agency CMOs	0	0
Derivative - interest rate product	0	0
Recurring [Member] \| Level 2 Inputs [Member]
Trading securities (Registered Mutual Funds) [Abstract]
Domestic Equity Mutual Funds	0	0
International Equity Mutual Funds	0	0
Fixed Income Mutual Funds	0	0
Registered Mutual Funds [Abstract]
Domestic Equity Mutual Funds	0	0
International Equity Mutual Funds	0	0
Fixed Income Mutual Funds	0	0
Pass-through MBS issued by GSEs	340,879	372
Agency CMOs	10,505	3,186
Derivative - interest rate product	4,041	3,228
Recurring [Member] \| Level 3 Inputs [Member]
Trading securities (Registered Mutual Funds) [Abstract]
Domestic Equity Mutual Funds	0	0
International Equity Mutual Funds	0	0
Fixed Income Mutual Funds	0	0
Registered Mutual Funds [Abstract]
Domestic Equity Mutual Funds	0	0
International Equity Mutual Funds	0	0
Fixed Income Mutual Funds	0	0
Pass-through MBS issued by GSEs	0	0
Agency CMOs	0	0
Derivative - interest rate product	0	0
Nonrecurring [Member]
Registered Mutual Funds [Abstract]
Assets measured at fair value	0	0
Financial Liabilities [Abstract]
Liabilities measured at fair value	$ 0	$ 0

No definition available.

Name:	dcom_AgencyCollateralizedMortgageObligationsFairValue
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Registered Domestic Equity Mutual Funds categorized as Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

No definition available.

+ Details

Name:	dcom_AvailableForSaleSecuritiesDomesticEquityMutualFundFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Registered Fixed Income Mutual Funds categorized as Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

No definition available.

+ Details

Name:	dcom_AvailableForSaleSecuritiesFixedIncomeMutualFundFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Registered International Equity Mutual Funds categorized as Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

No definition available.

+ Details

Name:	dcom_AvailableForSaleSecuritiesInternationalEquityMutualFundFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	dcom_PassThroughMbsIssuedByGsesFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

+ References

No definition available.

+ Details

Name:	dcom_TradingSecuritiesDomesticEquityMutualFundFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	dcom_TradingSecuritiesFixedIncomeMutualFundFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	dcom_TradingSecuritiesInternationalEquityMutualFundFairValueDisclosure
Namespace Prefix:	dcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ 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 (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: