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Significant Accounting Policies - Assets under Management (AUM) Subject to Incentive Fees and Incentive Fees (Detail) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Principal Transaction Revenue [Line Items] |  |
AUM Subject to Incentive Fees | $ 553,269,970 dhil_AssetsUnderManagementIncentiveFees | $ 482,095,096 dhil_AssetsUnderManagementIncentiveFees
Incentive Fees Under Method 2 |  |
Principal Transaction Revenue [Line Items] |  |
Performance Incentive Fees | 1,043,630 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember | 4,647,204 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember
Incentive Fees Under Method 2 | Quarter Ended June 30, 2017 |  |
Principal Transaction Revenue [Line Items] |  |
Performance Incentive Fees | 910,470 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember / us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis = dhil_AssetUnderManagementContractualPeriodEndDateMember | 4,647,204 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember / us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis = dhil_AssetUnderManagementContractualPeriodEndDateMember
Incentive Fees Under Method 2 | Quarter Ended December 31, 2018 |  |
Principal Transaction Revenue [Line Items] |  |
Performance Incentive Fees | 0 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember / us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis = dhil_AssetUnderManagementContractualPeriodEndDate2Member | 0 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember / us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis = dhil_AssetUnderManagementContractualPeriodEndDate2Member
Incentive Fees Under Method 2 | Quarter Ended September 30, 2019 |  |
Principal Transaction Revenue [Line Items] |  |
Performance Incentive Fees | $ 133,160 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember / us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis = dhil_AssetUnderManagementContractualPeriodEndDate3Member | $ 0 us-gaap_PerformanceFees / us-gaap_IncomeStatementLocationAxis = dhil_IncentiveFeesMethodTwoMember / us-gaap_PrincipleTransactionRevenueDescriptionOfReportingCategoryAxis = dhil_AssetUnderManagementContractualPeriodEndDate3Member

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

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- Definition Amount of revenue earned by the entity based on the investment results achieved. Excludes fees based upon a percentage of the assets under management not performance-related. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13 (a)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
Name: | us-gaap_PerformanceFees
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- 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_IncomeStatementLocationAxis=dhil_IncentiveFeesMethodTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |