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Investment Portfolio - Summary of Cash and Cash Equivalent Investments, Excluding Money Market Funds (Details) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Summary of market value of investments |  |
Total Investment Portfolio | $ 45,427,456 us-gaap_Investments | $ 18,726,070 us-gaap_Investments
Investments in Deferred Compensation Plans [Member] |  |
Summary of market value of investments |  |
Total Investment Portfolio | 5,678,764 us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_InvestmentsinDeferredCompensationPlansMember | 1,287,745 us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_InvestmentsinDeferredCompensationPlansMember
Seed Capital Investments [Member] |  |
Summary of market value of investments |  |
Total Investment Portfolio | 39,748,692 us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 17,438,325 us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill Research Opportunities Fund |  |
Summary of market value of investments |  |
Total Investment Portfolio | 13,143,281 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_ResearchOpportunitiesFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 13,305,830 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_ResearchOpportunitiesFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill Mid Cap Fund |  |
Summary of market value of investments |  |
Total Investment Portfolio | 10,729,930 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_MidCapFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 0 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_MidCapFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill Financial Long-Short Fund |  |
Summary of market value of investments |  |
Total Investment Portfolio | 1,222,892 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_FinancialLongShortFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 1,131,034 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_FinancialLongShortFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill High Yield Fund, L.P. |  |
Summary of market value of investments |  |
Total Investment Portfolio | 9,764,814 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_HIghYieldFundLPMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 0 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_HIghYieldFundLPMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill Valuation-Weighted 500, L.P. |  |
Summary of market value of investments |  |
Total Investment Portfolio | 3,398,035 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_DiamondHillValuationWeightedFiveHundredLpMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 3,000,328 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_DiamondHillValuationWeightedFiveHundredLpMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill Global Fund, L.P. |  |
Summary of market value of investments |  |
Total Investment Portfolio | 1,489,740 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_GlobalFundLPMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | 0 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_GlobalFundLPMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember
Seed Capital Investments [Member] | Diamond Hill Investment Partners, L.P. |  |
Summary of market value of investments |  |
Total Investment Portfolio | $ 0 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_DiamondHillInvestmentPartnersLpMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember | $ 1,133 us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_DiamondHillInvestmentPartnersLpMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember

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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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