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Significant Accounting Policies - Summary of Assets Under Management Subject to Fees (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($)
Quarter Ending September 30, 2021 |
Principal Transaction Revenue [Line Items] |
AUM subject to performance-based fees | $ 339,721,553
Unearned performance-based fees | 9,228,080
Quarter Ending December 31, 2021 |
Principal Transaction Revenue [Line Items] |
AUM subject to performance-based fees | 60,887,198
Unearned performance-based fees | 0
Total |
Principal Transaction Revenue [Line Items] |
AUM subject to performance-based fees | 400,608,751
Unearned performance-based fees | $ 9,228,080

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