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Operating Leases - Narrative (Detail) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
sqft
Location
Dec. 31, 2014
Dec. 31, 2015
Operating Leased Assets [Line Items]      
Area of operating lease 42,400dhil_LesseeLeasingAgreementsOperatingLeasesAreaunderLease    
Operating Lease Expense Excluding Rent   $ 300,000dhil_OperatingLeaseExpenseExcludingRent  
Number of office space locations 2us-gaap_PropertySubjectToOrAvailableForOperatingLeaseNumberOfUnits    
Total future lease payments due 6,107,000us-gaap_OperatingLeasesFutureMinimumPaymentsDue    
Contractual Reimbursement of Lease Payments Expected Over Life of Lease 300,000dhil_ContractualReimbursementofFutureMinimumLeasePaymentsExpectedOverLifeofOperatingLeaseAgreement    
Lease Forecast      
Operating Leased Assets [Line Items]      
Operating Lease Expense Excluding Rent     $ 400,000dhil_OperatingLeaseExpenseExcludingRent
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember