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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 20, 2020
Jun. 30, 2019
Cover page.      
Document Transition Report false    
Document Annual Report true    
Entity Interactive Data Current Yes    
Title of 12(b) Security Common shares, no par value    
City Area Code 614    
Entity Address, Postal Zip Code 43215    
Entity Incorporation, State or Country Code OH    
Entity File Number 000-24498    
Entity Registrant Name DIAMOND HILL INVESTMENT GROUP, INC    
Entity Address, Address Line One 325 John H. McConnell Blvd    
Entity Address, Address Line Two Suite 200    
Entity Address, City or Town Columbus    
Entity Address, State or Province OH    
Entity Central Index Key 0000909108    
Current Fiscal Year End Date --12-31    
Entity Filer Category Accelerated Filer    
Document Type 10-K    
Entity Small Business false    
Entity Emerging Growth Company false    
Document Period End Date Dec. 31, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   3,289,865  
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Shell Company false    
Entity Public Float     $ 468,216,662
Entity Tax Identification Number 65-0190407    
Local Phone Number 255-3333    
Trading Symbol DHIL    
Security Exchange Name NASDAQ    
Documents Incorporated by Reference
Portions of the registrant’s definitive Proxy Statement for the 2020 Annual Meeting of Shareholders to be filed pursuant to Regulation 14A under the Securities Exchange Act of 1934, as amended, are incorporated by reference into Part III of this Annual Report on Form 10-K.