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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, 2017 USD ($)
Quarter Ended December 31, 2018 |
Principal Transaction Revenue [Line Items] |
AUM subject to variable fees | $ 107,864,155
Unearned variable fees | 1,379,545
Quarter Ended September 30, 2019 |
Principal Transaction Revenue [Line Items] |
AUM subject to variable fees | 34,040,712
Unearned variable fees | 495,418
Quarter Ended March 31, 2020 |
Principal Transaction Revenue [Line Items] |
AUM subject to variable fees | 11,680,413
Unearned variable fees | 0
Quarter Ended September 30, 2021 |
Principal Transaction Revenue [Line Items] |
AUM subject to variable fees | 264,626,823
Unearned variable fees | 1,971,752
Total |
Principal Transaction Revenue [Line Items] |
AUM subject to variable fees | 418,212,103
Unearned variable fees | $ 3,846,715

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- Definition Assets under management (AUM) subject to variable performance fees and variable performance fees. + References No definition available. + Details
Name: | dhil_AssetsUnderManagementVariablePerformanceFees
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
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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- Details
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Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |