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Investment Portfolio Schedule of Equity Method Investment Ownership (Details)	Sep. 30, 2018
Research Opportunities Fund
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	30.00%
International Equity Fund LP
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	30.00%

X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- 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

X

- Details

Name:	us-gaap_InvestmentTypeAxis=dhil_ResearchOpportunitiesFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=dhil_InternationalEquityFundLPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: