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Significant Accounting Policies - Additional Information On Fair Value (Detail) (Level 1 Inputs [Member], Mutual Funds [Member], Money Market Funds [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Level 1 Inputs [Member] | Mutual Funds [Member] | Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalent fair value	$ 22.0	$ 4.1

X

- Definition

Cash equivalent fair value.

+ References

No definition available.

+ Details

Name:	dhil_CashEquivalentFairValue
Namespace Prefix:	dhil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration