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Significant Accounting Policies - Assets under Management (AUM) Subject to Incentive Fees and Incentive Fees (Detail) (Quarter Ended June 30, 2017 [Member], USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Principal Transaction Revenue [Line Items]
AUM Subject to Incentive Fees	$ 351,396,699	$ 273,834,389	$ 351,396,699	$ 273,834,389
Incentive Fees Under Method 1 [Member]
Principal Transaction Revenue [Line Items]
Performance Incentive Fees
Incentive Fees Under Method 2 [Member]
Principal Transaction Revenue [Line Items]
Performance Incentive Fees	$ 3,155,630	$ 53,600	$ 3,155,630	$ 53,600

X

- Definition

Assets under management (AUM) subject to incentive fees and incentive fees.

+ References

No definition available.

+ Details

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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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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Period Type:	duration