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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
Summary of market value of investments
Total Investment Portfolio	$ 45,427,456us-gaap_Investments	$ 18,726,070us-gaap_Investments
Investments in Deferred Compensation Plans [Member]
Summary of market value of investments
Total Investment Portfolio	5,678,764us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_InvestmentsinDeferredCompensationPlansMember	1,287,745us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_InvestmentsinDeferredCompensationPlansMember
Seed Capital Investments [Member]
Summary of market value of investments
Total Investment Portfolio	39,748,692us-gaap_Investments / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	17,438,325us-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	13,143,281us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_ResearchOpportunitiesFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	13,305,830us-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	10,729,930us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_MidCapFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	0us-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	1,222,892us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_FinancialLongShortFundMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	1,131,034us-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	9,764,814us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_HIghYieldFundLPMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	0us-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	3,398,035us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_DiamondHillValuationWeightedFiveHundredLpMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	3,000,328us-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	1,489,740us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_GlobalFundLPMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	0us-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	$ 0us-gaap_Investments / invest_InvestmentHoldingAxis = dhil_DiamondHillInvestmentPartnersLpMember / us-gaap_InvestmentTypeAxis = dhil_SeedCapitalInvestmentsMember	$ 1,133us-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

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