Operating Lease - Narrative (Detail) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) ft² location
Leases [Abstract]
Area of operating lease \| ft²	37,829
Number of office space locations \| location	1
Operating lease, right-of-use asset	$ 2.4
Operating lease, liability	$ 3.0

Lessee Leasing Agreements, Operating Leases, Area under Lease

No definition available.

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- Definition

Lessee Leasing Agreements, Operating Leases, locations

+ References

No definition available.

+ Details

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Namespace Prefix:	dhil_
Data Type:	xbrli:integerItemType
Balance Type:	na
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- References

No definition available.

+ Details

Name:	us-gaap_LeasesAbstract
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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

Name:	us-gaap_OperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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Balance Type:	debit
Period Type:	instant