Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements as of September 30, 2018 and December 31, 2017, and for the three- and nine-month periods ended September 30, 2018 and 2017, for Diamond Hill Investment Group, Inc. and its subsidiaries (referred to in these notes to the condensed consolidated financial statements as "the Company," "management," "we," "us," and "our") have been prepared in accordance with United States generally accepted accounting principles ("GAAP") and with the instructions to Form 10-Q and Article 10 of the Securities and Exchange Commission ("SEC") Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, all adjustments, consisting of normal recurring accruals, considered necessary for a fair statement of the financial condition and results of operations at the dates and for the interim periods presented, have been included. The results of operations for any interim period are not necessarily indicative of the results of operations to be expected for any full fiscal year. These unaudited condensed consolidated financial statements and footnotes should be read in conjunction with the audited consolidated financial statements of the Company included in the Company's Annual Report on Form 10-K for the fiscal year ended December 31, 2017 ("2017 Annual Report") as filed with the SEC.
Use of Estimates	Use of Estimates The preparation of the condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions related to the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the period. Actual results could differ 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 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 is an individual series of ETF Series Solutions which is also an open-end investment company registered under the 1940 Act. Each of the individual mutual funds and the ETF represents a separate share class of a legal entity organized under the Trust. The Company performs its analysis at the individual mutual fund and ETF level and has concluded the mutual funds and ETF are 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 ETF, the Diamond Hill Core Bond Fund, the Diamond Hill High Yield Fund, and the Diamond Hill Global Fund (collectively the "Consolidated Funds") as of September 30, 2018. DHCM is the managing member of Diamond Hill General Partner, LLC (the “General Partner”), which is the general partner of Diamond Hill Investment Partners, L.P. (“DHIP”), and Diamond Hill International Equity Fund, L.P. ("DHIEF"), each a limited partnership (collectively, the "Partnerships" or “LPs”) whose underlying assets consist primarily of marketable securities. DHCM is wholly owned by the Company and is consolidated by us. Further, DHCM, through its control of the General Partner, has the power to direct each LP’s economic activities and the right to receive investment advisory fees that may be significant to the LPs. The Company concluded it did not have a variable interest in DHIP as the fees paid to the General Partner are considered to contain customary terms and conditions as found in the market for similar products and the Company has no equity ownership in DHIP. The Company concluded DHIEF was a variable interest entity ("VIE") as DHCM has disproportionately less voting interest than economic interest, given that the limited partners have full power to remove the Company as the General Partner due to the existence of substantive kick-out rights. In addition, substantially all of DHIEF's activities are conducted on behalf of the General Partner which has disproportionately few voting rights. The Company concluded it is not the primary beneficiary of DHIEF as we lack the power to control the entity due to the existence of single-party kick-out rights where the limited partners have the unilateral ability to remove the General Partner without cause. DHCM’s investments in DHIEF are reported as a component of the Company’s investment portfolio, valued at DHCM’s respective share of the net income or loss of DHIEF. The LPs are not subject to lock-up periods and can be redeemed on demand. Gains and losses attributable to changes in the value of DHCM’s interests in the LPs are included in the Company’s reported investment income. The Company’s exposure to loss as a result of its involvement with the LPs is limited to the amount of its investments. DHCM is not obligated to provide, and has not provided, financial or other support to the LPs, other than its investments to date and its contractually provided investment advisory responsibilities. The Company has not provided liquidity arrangements, guarantees or other commitments to support the LPs’ operations, and the LPs’ creditors and interest holders have no recourse to the general credit of the Company. Certain board members and employees of the Company invest in the LPs and are not subject to a management fee or an incentive fee. These individuals receive no remuneration as a result of their personal investment in the LPs. The capital of the General Partner is not subject to a management fee or an incentive fee.
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, institutional accounts, and private investment funds. Therefore, no disclosures relating to operating segments are presented in the Company's annual or interim 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.
Fair Value Measurements	Fair Value Measurements Accounting Standards Codification Topic 820, Fair Value Measurement ("ASC 820") specifies a hierarchy of valuation classifications based on whether the inputs to the valuation techniques used in each valuation classification are observable or unobservable. These classifications are summarized in the three broad levels listed below: Level 1 - Unadjusted quoted prices for identical instruments in active markets. Level 2 - Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-driven valuations in which all significant inputs are observable. Level 3 - Valuations derived from techniques in which significant inputs are unobservable. Inputs used to measure fair value might fall in different levels of the fair value hierarchy, in which case the Company defaults to the lowest level input that is significant to the fair value measurement in its entirety. These levels are not necessarily an indication of the risk or liquidity associated with investments. Level 1 investments are comprised of investments in registered investment companies (mutual funds) or equity securities held in the Consolidated Funds and $98.9 million of investments in money market mutual funds owned by DHCM that the Company classifies as cash equivalents. Level 2 investments are comprised of investments in foreign equity securities and debt securities held in the Consolidated Funds, which are valued by an independent pricing service using pricing techniques which take into account factors such as trading activity, readily available market quotations, yield, quality, coupon rate, maturity, type of issue, trading characteristics, call features, credit rates and other observable inputs. The Company determines transfers between fair value hierarchy levels at the end of the reporting period. There were no transfers in or out of the levels during the nine months ended September 30, 2018. Changes in fair values of the investments are recorded in the Company's consolidated statements of income as investment income (loss), net.
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. Depreciation is calculated using the straight-line method over the estimated lives of the assets.
Revenue Recognition - General	New Accounting Guidance In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2014-09, "Revenue from Contracts with Customers", which supersedes existing accounting standards for revenue recognition and creates a single framework. ASU 2014-09 requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. This standard also specifies the accounting for certain costs to obtain or fulfill a contract with a customer. Our implementation efforts included a detailed review of revenue contracts within the scope of the guidance and an evaluation of the impact on the Company's revenue recognition policies. No transition-related practical expedients were applied. The Company adopted this ASU on its effective date, January 1, 2018, and it had no impact on the timing of the Company's revenue recognition. 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 assets under management ("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.
Revenue Recognition - Investment Advisory and Variable Incentive Revenue	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. 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 - Mutual Fund Administration	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 including 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.
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. As of September 30, 2018, the Company has recorded approximately $0.8 million for uncertain tax positions in the state and city jurisdictions in which we do business. The Company records interest and penalties within income tax expense on the income statement.
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 8.
Newly Issued But Not Yet Adopted Accounting Guidance	Newly Issued But Not Yet Adopted Accounting Guidance In February 2016, the FASB issued ASU 2016-02, "Leases", which, among other things, requires lessees to recognize most leases on-balance sheet. This will increase the reported assets and liabilities of lessees - in some cases significantly. Lessor accounting remains substantially similar to current GAAP. ASU 2016-02 supersedes Topic 840, Leases. ASU 2016-02 is effective for annual and interim periods in fiscal years beginning after December 15, 2018. ASU 2016-02 mandates a modified retrospective transition method for all entities. We will adopt this standard on its effective date, January 1, 2019. While we continue evaluating the full impact this standard will have on our consolidated financial statements, we expect the most significant impact will be the recognition of a lease liability and right of use asset on our consolidated balance sheets for our office operating lease. In August 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurements.” This update makes certain removals from, changes to and additions 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.