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Significant Accounting Policies - Assets under Management (AUM) Subject to Incentive Fees and Incentive Fees (Detail) | 3 Months Ended
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Mar. 31, 2021 USD ($)
Quarter Ending December 31, 2019 |
Principal Transaction Revenue [Line Items] |
AUM subject to performance-based fees | $ 67,514,790
Unearned performance-based fees | 125,672
Quarter Ending September 30, 2021 |
Principal Transaction Revenue [Line Items] |
AUM subject to performance-based fees | 375,886,495
Unearned performance-based fees | 9,928,373
Total |
Principal Transaction Revenue [Line Items] |
AUM subject to performance-based fees | 443,401,285
Unearned performance-based fees | $ 10,054,045

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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: |