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Operating Leases - Textual (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) ft² Location	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Leases [Abstract]
Lessee leasing agreements, operating leases, area under lease (in sqft) | ft²	37,829
Number of office space locations | Location	1
Operating lease expense excluding rent	$ 0.4	$ 0.4	$ 0.4
Operating Lease, Right-of-Use Asset	2.5			$ 2.9
Operating Lease, Liability	$ 3.1			$ 3.6

X

- Definition

Lessee Leasing Agreements, Operating Leases, Area under Lease

+ References

No definition available.

+ Details

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X

- Definition

Operating Lease Expense Excluding Rent

+ References

No definition available.

+ Details

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Namespace Prefix:	dhil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_LeasesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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X

- 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

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

X

- 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

X

- Definition

The number of units (items of property) under operating lease arrangements.

+ References

No definition available.

+ Details

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