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Compensation Plans - Schedule of Grants Issued and Grant Date Fair Value (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Grant date value $ 3,826,458us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation $ 3,984,817us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Shares Outstanding [Member]    
Shares issued 27,192us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementEquityComponentsAxis
= dhil_SharesOutstandingMember
33,745us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementEquityComponentsAxis
= dhil_SharesOutstandingMember
Common Stock [Member]    
Grant date value $ 3,826,458us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
$ 3,984,817us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember