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Operating Leases - Textual (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) ft² Location | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Leases [Abstract] |  |  |
Lessee leasing agreements, operating leases, area under lease (in sqft) | ft² | 37,829 |  |
Number of office space locations | Location | 1 |  |
Operating lease expense excluding rent | $ | $ 0.4 | $ 0.4 | $ 0.4

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- Definition Lessee Leasing Agreements, Operating Leases, Area under Lease + References No definition available. + Details
Name: | dhil_LesseeLeasingAgreementsOperatingLeasesAreaunderLease
Namespace Prefix: | dhil_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | duration

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- Definition Operating Lease Expense Excluding Rent + References No definition available. + Details
Name: | dhil_OperatingLeaseExpenseExcludingRent
Namespace Prefix: | dhil_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_LeasesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The number of units (items of property) under operating lease arrangements. + References No definition available. + Details
Name: | us-gaap_PropertySubjectToOrAvailableForOperatingLeaseNumberOfUnits
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant