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Significant Accounting Policies - Assets under Management (AUM) Subject to Incentive Fees and Incentive Fees (Detail) | 9 Months Ended
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Sep. 30, 2020 USD ($)
Quarter Ending December 31, 2019 |
Principal Transaction Revenue [Line Items] |
AUM subject to variable rate fees | $ 61,439,470
Unearned variable rate fees | 470,319
Quarter Ending September 30, 2021 |
Principal Transaction Revenue [Line Items] |
AUM subject to variable rate fees | 294,745,586
Unearned variable rate fees | 8,583,508
Total |
Principal Transaction Revenue [Line Items] |
AUM subject to variable rate fees | 356,185,056
Unearned variable rate fees | $ 9,053,827

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- Definition Assets under management (AUM) subject to variable rate fees and variable rate fees. + References No definition available. + Details
Name: | dhil_AssetsUnderManagementVariableRateFees
Namespace Prefix: | dhil_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Potential Variable Fees Based Upon Current Investment Results + References No definition available. + Details
Name: | dhil_PotentialVariableFeesBasedUponCurrentInvestmentResults
Namespace Prefix: | dhil_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_PrincipleTransactionRevenueLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis=dhil_AssetUnderManagementContractualPeriodEndDateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis=dhil_AssetUnderManagementContractualPeriodEndDate4Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis=dhil_AssetUnderManagementContractualPeriodEndTotalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |