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Investment Portfolio - Summary of Market Value of Investments (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Total Investment Portfolio	$ 22,083,216	$ 16,503,731
Diamond Hill Small Cap Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	279,788	215,550
Diamond Hill Small-Mid Cap Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	310,929	239,316
Diamond Hill Large Cap Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	308,579	246,744
Diamond Hill Select Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	317,971	242,252
Diamond Hill Long-Short Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	1,201,473	1,036,045
Diamond Hill Financial Long Short Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	2,045,949
Diamond Hill Research Opportunities Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	13,166,502	10,658,665
Diamond Hill Strategic Income Fund [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	212,480	214,598
Diamond Hill Investment Partners, L.P. [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	9,075	7,336
Diamond Hill Research Partners-International, L.P. [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	1,530,093	1,384,976
Diamond Hill Valuation-Weighted 500, L.P. [Member]
Investment Holdings [Line Items]
Total Investment Portfolio	$ 2,700,377	$ 2,258,249