Operating Leases - Narrative (Detail) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018 ft² location	Dec. 31, 2017 USD ($)
Leases [Abstract]
Area of operating lease \| ft²	37,829
Number of office space locations \| location	1
Operating lease expense excluding rent \| $		$ 0.4

Lessee Leasing Agreements, Operating Leases, Area under Lease

No definition available.

Name:	dhil_LesseeLeasingAgreementsOperatingLeasesAreaunderLease
Namespace Prefix:	dhil_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	duration