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PENSION AND OTHER POSTRETIREMENT PLANS - 401k Plan (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
PENSION AND OTHER POSTRETIREMENT PLANS. |  |  |
Participants contribution | $ 19,500 |  |
Description of Defined Contribution Pension and Other Postretirement Plans | 100% of each employee’s contributions up to 1% of each employee’s compensation plus 50% of each employee’s contributions over 1% but not in excess of 6% of each employee’s compensation for a maximum contribution of 3.5% of a participating employee’s compensation |  |
Minimum employee contribution percentage | 1.00% |  |
Percentage of employer matching contribution | 50.00% |  |
Threshold limit percentage of employee compensation | 6.00% |  |
Maximum percentage of annual contribution per employee | 3.50% |  |
Cash contributions by the Bank | $ 1,400,000 | $ 1,100,000 | $ 1,000,000.0
Discretionary profit sharing contribution | $ 546,000 | $ 583,000 | $ 497,000

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- Definition Minimum percentage of employee gross pay, by the terms of the plan, that the employer may contribute to a defined contribution plan. + References No definition available. + Details
Name: | dcom_DefinedContributionPlanMinimumAnnualContributionPerEmployeePercent
Namespace Prefix: | dcom_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the threshold limit in percentage of employees' compensation eligible for employer matching contributions. + References No definition available. + Details
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Namespace Prefix: | dcom_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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Balance Type: | debit
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X
- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
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Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum amount the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeeAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Description of defined contribution pension and other postretirement plans. + References No definition available. + Details
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