Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying Consolidated Financial Statements of the Company have been prepared pursuant to the rules and regulations of the U. S. Securities and Exchange Commission (“SEC”) and in accordance with the instructions to Form 10-K. The Company believes that the disclosures contained herein are adequate to make the information presented not misleading. These Consolidated Financial Statements reflect, in the opinion of the Company, all material adjustments (which include only normal recurring adjustments) necessary to fairly present the Company’s financial position as of December 31, 2019 and 2018, and results of operations for the years ended December 31, 2019, 2018 and 2017. The preparation of the Consolidated Financial Statements requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements as well as the reported amounts of revenue and expense during the reporting period. Estimates have been prepared on the basis of the most current and best available information, but actual results could differ materially from those estimates.
Reclassification	Reclassification Certain prior period amounts and disclosures may have been reclassified to conform to the current period’s financial presentation.
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the operations of the Company and its controlled subsidiaries. All inter-company transactions and balances have been eliminated in consolidation. The Company holds certain investments in the Funds, and previously held an investment in the ETF, for general corporate investment purposes, to provide seed capital for newly formed strategies or to add capital to existing strategies. The Funds are organized in a series fund structure in which there are multiple mutual funds within one Trust. The Trust is an open-end investment company registered under the Investment Company Act of 1940, as amended (the"1940 Act"). The ETF was an individual series of ETF Series Solutions, which was also an open-end investment company registered under the 1940 Act. The ETF liquidated and its assets were distributed to its shareholders on April 5, 2019. Each of the individual mutual funds represents (and the ETF represented) a separate share class of a legal entity organized under the Trust. The Company performs its consolidation analysis at the individual mutual fund and ETF level and has concluded the mutual funds are, and the ETF was, voting rights entities ("VREs") because the structure of the investment product is such that the shareholders are deemed to have the power through voting rights to direct the activities that most significantly impact the entity's economic performance. To the extent material, these investment products are consolidated if Company ownership, directly or indirectly, represents a majority interest (greater than 50%). The Company records redeemable noncontrolling interests in consolidated investments for which the Company's ownership is less than 100%. The Company has consolidated the Diamond Hill International Fund and the Diamond Hill Global Fund (collectively the "Consolidated Funds") as of December 31, 2019. The Company deconsolidated the ETF, the Diamond Hill Core Bond Fund and the Diamond Hill High Yield Fund during the year ended December 31, 2019 as the Company's ownership declined to less than 50%.
Redeemable Noncontrolling Interest	Redeemable Noncontrolling Interest Redeemable noncontrolling interest represents third-party interests in the Consolidated Funds. This interest is redeemable at the option of the investors and therefore is not treated as permanent equity. Redeemable noncontrolling interest is recorded at redemption value, which approximates the fair value each reporting period.
Segment Information	Segment Information Management has determined that the Company operates in one business segment, providing investment management and administration services to mutual funds, sub-advised mutual funds, and separately managed accounts. Therefore, no disclosures relating to operating segments are presented in the annual financial statements.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include demand deposits and money market mutual funds held by DHCM.
Accounts Receivable	Accounts Receivable Accounts receivable are recorded when they are due and are presented on the balance sheet net of any allowance for doubtful accounts. Accounts receivable are written off when they are determined to be uncollectible. Any allowance for doubtful accounts is estimated based on the Company’s historical losses, existing conditions in the industry, and the financial stability of the individuals or entities that owe the receivable.
Investments	Investments Management determines the appropriate classification of its investments at the time of purchase and re-evaluates its determination at each reporting period. Investments in the Funds we advise where the Company has neither control nor the ability to exercise significant influence, as well as securities held in the Consolidated Funds, are measured at fair value based on quoted market prices. Unrealized gains and losses are recorded as investment income (loss) in the Company’s consolidated statements of income. Investments classified as equity method investments represent investments in which the Company owns between 20-50% of the outstanding voting interests in the entity or when it is determined that the Company is able to exercise significant influence but not control over the investments. When using the equity method, the Company recognizes its respective share of the investee’s net income or loss for the period which is recorded as investment income in the Company’s consolidated statements of income.
Property and Equipment	Property and Equipment Property and equipment, consisting of leasehold improvements, computer equipment, furniture, and fixtures, are carried at cost less accumulated depreciation. Accumulated depreciation was $6.4 million and $5.2 million as of December 31, 2019 and 2018, respectively. Depreciation is calculated using the straight-line method over the estimated lives of the assets.
Revenue Recognition	Revenue Recognition – Mutual Fund Administration DHCM has an administrative and transfer agency services agreement with the Funds under which DHCM performs certain services for each Fund. These services include performance obligations, such as mutual fund administration, fund accounting, transfer agency, and other related functions. These services are performed concurrently under our agreement with the Funds, and all performance obligations to provide these administrative services are satisfied over time, and the Company recognizes revenue as time passes. For performing these services, each Fund pays DHCM a fee, which is calculated using an annual rate times the average daily net assets of each respective share class. These fees are thereby constrained and represent variable consideration, and are excluded from revenue until the AUM on which we bill the Funds is no longer subject to market fluctuations. The Funds have selected and contractually engaged certain vendors to fulfill various services to benefit the Funds’ shareholders or to satisfy regulatory requirements of the Funds. These services include, among others, required shareholder mailings, federal and state registrations, and legal and audit services. DHCM, in fulfilling a portion of its role under the administration agreement with the Funds, acts as agent to pay these obligations of the Funds. Each vendor is independently responsible for fulfillment of the services it has been engaged to provide and negotiates fees and terms with the management and board of trustees of the Funds. The fee that each Fund pays to DHCM is reviewed annually by the Funds’ board of trustees and specifically takes into account the contractual expenses that DHCM pays on behalf of the Funds. As a result, DHCM is not involved in the delivery or pricing of these services and bears no risk related to these services. Revenue has been recorded net of these Fund related expenses. In addition, DHCM advances the upfront commissions that are paid to brokers who sell Class C shares of the Funds. These advances are capitalized and amortized over 12 months to correspond with the repayments DHCM receives from the principal underwriter to recoup this commission advancement. Revenue Recognition – Investment Advisory Fees The Company’s investment advisory contracts have a single performance obligation (the investment advisory services provided to the client) as the promised services are not separately identifiable from other promises in the contracts and, therefore, are not distinct. All performance obligations to provide advisory services are satisfied over time and the Company recognizes revenue as time passes. The fees we receive for our services under our investment advisory contracts are based on our AUM, which changes based on the value of securities held under each advisory contract. These fees are thereby constrained and represent variable consideration, and are excluded from revenue until the AUM on which our client is billed is no longer subject to market fluctuations. The Company also provides services to UMA programs in which an investment manager provides its strategy model portfolio to the sponsor of the UMA program. The Company is paid a portion of the UMA fee for its services by the program sponsor at a pre-determined rate based on assets in the program. UMA program revenues were $2.0 million, $1.5 million and $1.2 million as of December 31, 2019, 2018 and 2017, respectively and are included in investment advisory fees in the consolidated statements of income. Revenue Recognition – Variable Rate Fees The Company manages certain client accounts that provide for variable rate fees. These fees are calculated based on client investment results over rolling five-year periods. The Company records variable rate fees at the end of the contract measurement period because the variable fees earned are constrained based on movements in the financial markets. Revenue Recognition – General Revenue is recognized when performance obligations under the terms of a contract with a client are satisfied. The Company earns substantially all of its revenue from investment advisory and fund administration contracts. Investment advisory and administration fees, generally calculated as a percentage of AUM are recorded as revenue as services are performed. In addition to fixed fees based on a percentage of AUM, certain client accounts also provide periodic variable rate fees.
Income Taxes	Income Taxes The Company accounts for current and deferred income taxes through an asset and liability approach. Deferred tax assets are recognized for deductible temporary differences and deferred tax liabilities are recognized for taxable temporary differences. Deferred tax assets are reduced by a valuation allowance when it is more likely than not that some portion or all of the deferred tax assets will not be realized. Deferred tax assets and liabilities are adjusted for the effects of changes in tax laws and rates on the date of enactment. The Company is subject to examination by federal and applicable state and local jurisdictions for various tax periods. The Company’s income tax positions are based on research and interpretations of the income tax laws and rulings in each of the jurisdictions in which it does business. Due to the subjectivity of interpretations of laws and rulings in each jurisdiction, the differences and interplay in tax laws among those jurisdictions, and the inherent uncertainty in estimating the final resolution of complex tax audit matters, the Company’s estimates of income tax liabilities may differ from actual payments or assessments. The Company regularly assesses its position with regard to tax exposures and records liabilities for these uncertain tax positions and related interest and penalties, if any, according to the principles of FASB ASC 740, Income Taxes. The Company records interest and penalties within income tax expense on the income statement. See Note 9.
Earnings Per Share	Earnings Per Share Basic earnings per share (“EPS”) excludes dilution and is computed by dividing net income by the weighted average number of common shares outstanding for the period, which includes participating securities. Diluted EPS reflects the potential dilution of EPS due to unvested restricted stock units. See Note 10.
New and Newly Issued But Not Yet Adopted Accounting Guidance	Newly Issued But Not Yet Adopted Accounting Guidance In August 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurements.” This update makes certain revisions to existing disclosure requirements for fair value measurement. ASU 2018-13 does not change fair value measurements already required or permitted by existing standards. ASU 2018-13 is effective for financial statements issued for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Management does not believe that adoption of ASU 2018-13 will materially impact the Company’s financial statements.

Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

The accompanying Consolidated Financial Statements of the Company have been prepared pursuant to the rules and regulations of the U. S. Securities and Exchange Commission (“SEC”) and in accordance with the instructions to Form 10-K. The Company believes that the disclosures contained herein are adequate to make the information presented not misleading.

These Consolidated Financial Statements reflect, in the opinion of the Company, all material adjustments (which include only normal recurring adjustments) necessary to fairly present the Company’s financial position as of December 31, 2019 and 2018, and results of operations for the years ended December 31, 2019, 2018 and 2017. The preparation of the Consolidated Financial Statements requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements as well as the reported amounts of revenue and expense during the reporting period. Estimates have been prepared on the basis of the most current and best available information, but actual results could differ materially from those estimates.

Reclassification

Certain prior period amounts and disclosures may have been reclassified to conform to the current period’s financial presentation.

Principles of Consolidation

The accompanying consolidated financial statements include the operations of the Company and its controlled subsidiaries. All inter-company transactions and balances have been eliminated in consolidation.

The Company holds certain investments in the Funds, and previously held an investment in the ETF, for general corporate investment purposes, to provide seed capital for newly formed strategies or to add capital to existing strategies. The Funds are organized in a series fund structure in which there are multiple mutual funds within one Trust. The Trust is an open-end investment company registered under the Investment Company Act of 1940, as amended (the"1940 Act"). The ETF was an individual series of ETF Series Solutions, which was also an open-end investment company registered under the 1940 Act. The ETF liquidated and its assets were distributed to its shareholders on April 5, 2019. Each of the individual mutual funds represents (and the ETF represented) a separate share class of a legal entity organized under the Trust.

The Company performs its consolidation analysis at the individual mutual fund and ETF level and has concluded the mutual funds are, and the ETF was, voting rights entities ("VREs") because the structure of the investment product is such that the shareholders are deemed to have the power through voting rights to direct the activities that most significantly impact the entity's economic performance. To the extent material, these investment products are consolidated if Company ownership, directly or indirectly, represents a majority interest (greater than 50%). The Company records redeemable noncontrolling interests in consolidated investments for which the Company's ownership is less than 100%. The Company has consolidated the Diamond Hill International Fund and the Diamond Hill Global Fund (collectively the "Consolidated Funds") as of

December 31, 2019. The Company deconsolidated the ETF, the Diamond Hill Core Bond Fund and the Diamond Hill High Yield Fund during the year ended December 31, 2019 as the Company's ownership declined to less than 50%.

Redeemable Noncontrolling Interest

Redeemable noncontrolling interest represents third-party interests in the Consolidated Funds. This interest is redeemable at the option of the investors and therefore is not treated as permanent equity. Redeemable noncontrolling interest is recorded at redemption value, which approximates the fair value each reporting period.

Segment Information

Management has determined that the Company operates in one business segment, providing investment management and administration services to mutual funds, sub-advised mutual funds, and separately managed accounts. Therefore, no disclosures relating to operating segments are presented in the annual financial statements.

Cash and Cash Equivalents

Cash and cash equivalents include demand deposits and money market mutual funds held by DHCM.

Accounts Receivable

Accounts receivable are recorded when they are due and are presented on the balance sheet net of any allowance for doubtful accounts. Accounts receivable are written off when they are determined to be uncollectible. Any allowance for doubtful accounts is estimated based on the Company’s historical losses, existing conditions in the industry, and the financial stability of the individuals or entities that owe the receivable.

Investments

Management determines the appropriate classification of its investments at the time of purchase and re-evaluates its determination at each reporting period.

Investments in the Funds we advise where the Company has neither control nor the ability to exercise significant influence, as well as securities held in the Consolidated Funds, are measured at fair value based on quoted market prices. Unrealized gains and losses are recorded as investment income (loss) in the Company’s consolidated statements of income.

Investments classified as equity method investments represent investments in which the Company owns between 20-50% of the outstanding voting interests in the entity or when it is determined that the Company is able to exercise significant influence but not control over the investments. When using the equity method, the Company recognizes its respective share of the investee’s net income or loss for the period which is recorded as investment income in the Company’s consolidated statements of income.

Property and Equipment

Property and equipment, consisting of leasehold improvements, computer equipment, furniture, and fixtures, are carried at cost less accumulated depreciation. Accumulated depreciation was $6.4 million and $5.2 million as of December 31, 2019 and 2018, respectively. Depreciation is calculated using the straight-line method over the estimated lives of the assets.

Revenue Recognition

Revenue Recognition – Mutual Fund Administration

DHCM has an administrative and transfer agency services agreement with the Funds under which DHCM performs certain services for each Fund. These services include performance obligations, such as mutual fund administration, fund accounting, transfer agency, and other related functions. These services are performed concurrently under our agreement with the Funds, and all performance obligations to provide these administrative services are satisfied over time, and the Company recognizes revenue as time passes. For performing these services, each Fund pays DHCM a fee, which is calculated using an annual rate times the average daily net assets of each respective share class. These fees are thereby constrained and represent variable consideration, and are excluded from revenue until the AUM on which we bill the Funds is no longer subject to market fluctuations.

The Funds have selected and contractually engaged certain vendors to fulfill various services to benefit the Funds’ shareholders or to satisfy regulatory requirements of the Funds. These services include, among others, required shareholder mailings, federal and state registrations, and legal and audit services. DHCM, in fulfilling a portion of its role under the administration agreement with the Funds, acts as agent to pay these obligations of the Funds. Each vendor is independently responsible for fulfillment of the services it has been engaged to provide and negotiates fees and terms with the management and board of trustees of the Funds. The fee that each Fund pays to DHCM is reviewed annually by the Funds’ board of trustees and specifically takes into account the contractual expenses that DHCM pays on behalf of the Funds. As a result, DHCM is not involved in the delivery or pricing of these services and bears no risk related to these services. Revenue has been recorded net of these Fund related expenses. In addition, DHCM advances the upfront commissions that are paid to brokers who sell Class C shares of the Funds. These advances are capitalized and amortized over 12 months to correspond with the repayments DHCM receives from the principal underwriter to recoup this commission advancement.

Revenue Recognition – Investment Advisory Fees

The Company’s investment advisory contracts have a single performance obligation (the investment advisory services provided to the client) as the promised services are not separately identifiable from other promises in the contracts and, therefore, are not distinct. All performance obligations to provide advisory services are satisfied over time and the Company recognizes revenue as time passes.

The fees we receive for our services under our investment advisory contracts are based on our AUM, which changes based on the value of securities held under each advisory contract. These fees are thereby constrained and represent variable consideration, and are excluded from revenue until the AUM on which our client is billed is no longer subject to market fluctuations.

The Company also provides services to UMA programs in which an investment manager provides its strategy model portfolio to the sponsor of the UMA program. The Company is paid a portion of the UMA fee for its services by the program sponsor at a pre-determined rate based on assets in the program. UMA program revenues were $2.0 million, $1.5 million and $1.2 million as of December 31, 2019, 2018 and 2017, respectively and are included in investment advisory fees in the consolidated statements of income.

Revenue Recognition – Variable Rate Fees

The Company manages certain client accounts that provide for variable rate fees. These fees are calculated based on client investment results over rolling five-year periods. The Company records variable rate fees at the end of the contract measurement period because the variable fees earned are constrained based on movements in the financial markets.

Revenue Recognition – General

Revenue is recognized when performance obligations under the terms of a contract with a client are satisfied. The Company earns substantially all of its revenue from investment advisory and fund administration contracts. Investment advisory and administration fees, generally calculated as a percentage of AUM are recorded as revenue as services are performed. In addition to fixed fees based on a percentage of AUM, certain client accounts also provide periodic variable rate fees.

Income Taxes

The Company accounts for current and deferred income taxes through an asset and liability approach. Deferred tax assets are recognized for deductible temporary differences and deferred tax liabilities are recognized for taxable temporary differences. Deferred tax assets are reduced by a valuation allowance when it is more likely than not that some portion or all of the deferred tax assets will not be realized. Deferred tax assets and liabilities are adjusted for the effects of changes in tax laws and rates on the date of enactment.

The Company is subject to examination by federal and applicable state and local jurisdictions for various tax periods. The Company’s income tax positions are based on research and interpretations of the income tax laws and rulings in each of the jurisdictions in which it does business. Due to the subjectivity of interpretations of laws and rulings in each jurisdiction, the differences and interplay in tax laws among those jurisdictions, and the inherent uncertainty in estimating the final resolution of complex tax audit matters, the Company’s estimates of income tax liabilities may differ from actual payments or assessments. The Company regularly assesses its position with regard to tax exposures and records liabilities for these uncertain tax positions and related interest and penalties, if any, according to the principles of FASB ASC 740, Income Taxes. The Company records interest and penalties within income tax expense on the income statement. See Note 9.

Earnings Per Share

Basic earnings per share (“EPS”) excludes dilution and is computed by dividing net income by the weighted average number of common shares outstanding for the period, which includes participating securities. Diluted EPS reflects the potential dilution of EPS due to unvested restricted stock units. See Note 10.

New and Newly Issued But Not Yet Adopted Accounting Guidance

Newly Issued But Not Yet Adopted Accounting Guidance

In August 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurements.” This update makes certain revisions to existing disclosure requirements for fair value measurement. ASU 2018-13 does not change fair value measurements already required or permitted by existing standards. ASU 2018-13 is effective for financial statements issued for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Management does not believe that adoption of ASU 2018-13 will materially impact the Company’s financial statements.