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Investment Portfolio - Summary of Market Value of Investments (Detail) - USD ($) | Jun. 30, 2015 | Dec. 31, 2014
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Investment Holdings [Line Items] |  |
Total Investment Portfolio | $ 53,360,847 | $ 45,427,456
Seed Capital Investments |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 43,149,566 | 39,748,692
Seed Capital Investments | Diamond Hill Research Opportunities Fund |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 12,221,928 | 13,143,281
Seed Capital Investments | Diamond Hill Mid Cap Fund |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 11,229,859 | 10,729,930
Seed Capital Investments | Diamond Hill Financial Long-Short Fund |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 1,232,264 | 1,222,892
Seed Capital Investments | Diamond Hill Valuation- Weighted 500 ETF |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 6,644,763 | 0
Seed Capital Investments | Diamond Hill Valuation-Weighted 500, L.P. |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 0 | 3,398,035
Seed Capital Investments | Diamond Hill High Yield Fund, L.P. |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 10,238,133 | 9,764,814
Seed Capital Investments | Diamond Hill Global Fund, L.P. |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | 1,582,619 | 1,489,740
Investments in Deferred Compensation Plans |  |
Investment Holdings [Line Items] |  |
Total Investment Portfolio | $ 10,211,281 | $ 5,678,764

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