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INVESTMENT AND MORTGAGE-BACKED SECURITIES, Fiscal Year Maturity and Other Disclosures (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Sep. 01, 2008
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]        
Due after one year through three years $ 70dcom_DueAfterOneYearThroughThreeYears      
Amortized Cost, Total 70us-gaap_AvailableForSaleSecuritiesAmortizedCost      
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]        
Due after one year through three years, fair value 70dcom_DueAfterOneYearThroughThreeYearsFairValue      
Fair Value 70dcom_AvailableForSaleSecuritiesDebtFairValue      
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 3,780us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesEquity 366us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesEquity 22,415us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesEquity  
Net gain recognized on sale of investment securities available for sale 997us-gaap_GainLossOnSaleOfDebtInvestments 110us-gaap_GainLossOnSaleOfDebtInvestments 941us-gaap_GainLossOnSaleOfDebtInvestments  
Proceeds from the sales of mortgage backed securities available for sale 0us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale 0us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale 21,949us-gaap_ProceedsFromSaleOfMortgageBackedSecuritiesMBSCategorizedAsAvailableForSale  
Net gain recognized on sale of mortgage backed securities available for sale 0dcom_GainOnThesaleOfMortgageBackedSecurities 0dcom_GainOnThesaleOfMortgageBackedSecurities 81dcom_GainOnThesaleOfMortgageBackedSecurities  
Amortized cost of securities transferred       19,922dcom_AmortizedCostOfSecuritiesTransferred
Unrealized loss on securities transferred recognized in accumulated other comprehensive income loss net of tax       8,420dcom_UnrealizedLossOnSecuritiesTransferredRecognizedInAccumulatedOtherComprehensiveIncomeLossNetOfTax
Transfers [Member]        
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]        
Cumulative balance at the beginning of the period 997us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= dcom_TransfersMember
1,268us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= dcom_TransfersMember
   
Amortization of Transfer Loss (65)dcom_AmortizationOfTransferLoss
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= dcom_TransfersMember
(271)dcom_AmortizationOfTransferLoss
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= dcom_TransfersMember
   
Cumulative balance at end of the period 932us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= dcom_TransfersMember
997us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= dcom_TransfersMember
   
Pooled Trust Preferred Held To Maturity Securities [Member]        
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]        
Fair Value 6,315dcom_AvailableForSaleSecuritiesDebtFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
5,163dcom_AvailableForSaleSecuritiesDebtFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
   
Schedule Of Available For Sale And Held For Maturity Securities [Line Items]        
Securities Weighted Average Term To Maturity (in years) 20 years      
Number of securities deemed have OTTI 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
8,974dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
 
OTTI recognized on securities with previous OTTI   0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAdditionsAdditionalLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
181dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAdditionsAdditionalLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
 
Reductions and transfers to credit-related OTTI   0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
 
Amortization of previously recognized OTTI 0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
(210)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_CreditRelatedOttiMember
 
Cumulative balance at end 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
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 601dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
634dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
930dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
 
OTTI recognized on securities with previous OTTI   0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAdditionsAdditionalLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
6dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAdditionsAdditionalLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
 
Reductions and transfers to credit-related OTTI   0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
(181)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
 
Amortization of previously recognized OTTI (32)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
(33)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
(121)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_NonCreditRelatedOttiMember
 
Cumulative balance at end 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
634dcom_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 9,546dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
9,579dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
9,904dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
 
OTTI recognized on securities with previous OTTI   0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAdditionsAdditionalLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
187dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAdditionsAdditionalLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
 
Reductions and transfers to credit-related OTTI   0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
(181)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
 
Amortization of previously recognized OTTI (32)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
(33)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
(331)dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCIAmortizationOfOTTI
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_PooledTrustPreferredHeldToMaturitySecuritiesMember
/ dcom_OtherThanTemporaryImpairmentCategoryAxis
= dcom_TotalCreditAndNonCreditOttiMember
 
Cumulative balance at end 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
9,579dcom_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) 16 years 9 months 18 days      
Weighted average duration of mortgage backed securities available for sale 1 year 1 month 6 days      
Mutual Fund Investments [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 106dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
348dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
1,425dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
 
Reductions and transfers to credit-related OTTI (106)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
(242)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
(1,077)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
 
Cumulative balance at end of the period $ 0dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
$ 106dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember
$ 348dcom_OtherThanTemporaryImpairmentCreditAndNonCreditLossesRecognizedInEarningsAndOCILossesOnDebtSecuritiesHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= dcom_MutualFundInvestmentsMember