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

X

- 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

X

- 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

X

- Details

Name:	us-gaap_PrincipleTransactionRevenueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_IncomeStatementLocationAxis=dhil_IncentiveFeesMethodTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: