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INVESTMENT AND MORTGAGE-BACKED SECURITIES, Fiscal Year Maturity and Other Disclosures (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Other Than Temporary Impairment Credit And Non Credit Losses Recognized In Earnings And OCI [Roll Forward]    
Proceeds from sale of mortgage backed securities available for sale $ 24,307us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale $ 0us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale
Pooled Trust Preferred Held To Maturity Securities [Member]    
Schedule Of Available For Sale And Held For Maturity Securities [Line Items]    
Securities Weighted Average Term To Maturity (in years) 19 years 8 months 12 days  
Held To Maturity Securities With OTTI Number Of Securities 5dcom_HeldToMaturitySecuritiesWithOttiNumberOfSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
 
Pooled Trust Preferred Held To Maturity Securities [Member] | Credit Related Otti [Member]    
Other Than Temporary Impairment Credit And Non Credit Losses Recognized In Earnings And OCI [Roll Forward]    
Cumulative balance at the beginning of the period 8,945dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
8,945dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
Amortization of previously recognized OTTI (150)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
Cumulative balance at end of the period 8,795dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
8,945dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
Pooled Trust Preferred Held To Maturity Securities [Member] | Non Credit Related Otti [Member]    
Other Than Temporary Impairment Credit And Non Credit Losses Recognized In Earnings And OCI [Roll Forward]    
Cumulative balance at the beginning of the period 569dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
601dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
Amortization of previously recognized OTTI (8)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
(8)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
Cumulative balance at end of the period 561dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
593dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
Pooled Trust Preferred Held To Maturity Securities [Member] | Total Credit and Non Credit Otti [Member]    
Other Than Temporary Impairment Credit And Non Credit Losses Recognized In Earnings And OCI [Roll Forward]    
Cumulative balance at the beginning of the period 9,514dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
9,546dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
Amortization of previously recognized OTTI (158)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
(8)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
Cumulative balance at end of the period 9,356dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
9,538dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
All Mortgage Backed Securities available for sale [Member]    
Schedule Of Available For Sale And Held For Maturity Securities [Line Items]    
Securities Weighted Average Term To Maturity (in years) 12 years 10 months 24 days  
Other Than Temporary Impairment Credit And Non Credit Losses Recognized In Earnings And OCI [Roll Forward]    
Proceeds from sale of mortgage backed securities available for sale 24,307us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllMortgageBackedSecuritiesAvailableForSaleMember
0us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllMortgageBackedSecuritiesAvailableForSaleMember
Available-for-sale Securities, Gross Realized Gains 1,395us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllMortgageBackedSecuritiesAvailableForSaleMember
0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllMortgageBackedSecuritiesAvailableForSaleMember
Available-for-sale Securities, Gross Realized Losses 7us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllMortgageBackedSecuritiesAvailableForSaleMember
0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllMortgageBackedSecuritiesAvailableForSaleMember
All Investment Securities Available for sale [Member]    
Other Than Temporary Impairment Credit And Non Credit Losses Recognized In Earnings And OCI [Roll Forward]    
Proceeds from sale of investment securities available-for-sale 70us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesEquity
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllInvestmentSecuritiesAvailableForSaleMember
0us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesEquity
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllInvestmentSecuritiesAvailableForSaleMember
Available-for-sale Securities, Gross Realized Gains 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllInvestmentSecuritiesAvailableForSaleMember
0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllInvestmentSecuritiesAvailableForSaleMember
Available-for-sale Securities, Gross Realized Losses $ 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllInvestmentSecuritiesAvailableForSaleMember
$ 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_AllInvestmentSecuritiesAvailableForSaleMember