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Investment Portfolio - Summary of Market Value of Investments (Detail) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Investment Holdings [Line Items]    
Total Investment Portfolio $ 49,656,404us-gaap_Investments $ 45,427,456us-gaap_Investments
Seed Capital Investments    
Investment Holdings [Line Items]    
Total Investment Portfolio 39,568,878us-gaap_Investments
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 39,748,692us-gaap_Investments
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Seed Capital Investments | Diamond Hill Research Opportunities Fund    
Investment Holdings [Line Items]    
Total Investment Portfolio 12,076,180us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_ResearchOpportunitiesFundMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 13,143,281us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_ResearchOpportunitiesFundMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Seed Capital Investments | Diamond Hill Mid Cap Fund    
Investment Holdings [Line Items]    
Total Investment Portfolio 11,159,899us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_MidCapFundMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 10,729,930us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_MidCapFundMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Seed Capital Investments | Diamond Hill Financial Long-Short Fund    
Investment Holdings [Line Items]    
Total Investment Portfolio 1,211,644us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_DiamondHillValuationWeightedFiveHundredLpMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 1,222,892us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_DiamondHillValuationWeightedFiveHundredLpMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Seed Capital Investments | Diamond Hill High Yield Fund, L.P.    
Investment Holdings [Line Items]    
Total Investment Portfolio 10,113,107us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_GlobalFundLPMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 9,764,814us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_GlobalFundLPMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Seed Capital Investments | Diamond Hill Valuation-Weighted 500, L.P.    
Investment Holdings [Line Items]    
Total Investment Portfolio 3,437,731us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_FinancialLongShortFundMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 3,398,035us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_FinancialLongShortFundMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Seed Capital Investments | Diamond Hill Global Fund, L.P.    
Investment Holdings [Line Items]    
Total Investment Portfolio 1,570,317us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_DiamondHillInvestmentPartnersLpMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember		 1,489,740us-gaap_Investments
/ invest_InvestmentHoldingAxis
= dhil_DiamondHillInvestmentPartnersLpMember
/ us-gaap_InvestmentTypeAxis
= dhil_SeedCapitalInvestmentsMember
Investments in Deferred Compensation Plans    
Investment Holdings [Line Items]    
Total Investment Portfolio $ 10,087,526us-gaap_Investments
/ us-gaap_InvestmentTypeAxis
= dhil_InvestmentsinDeferredCompensationPlansMember		 $ 5,678,764us-gaap_Investments
/ us-gaap_InvestmentTypeAxis
= dhil_InvestmentsinDeferredCompensationPlansMember