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

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- Definition Sum of the carrying amounts as of the balance sheet date of all investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(h)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph h -Article 7 + Details
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