| Cover Page Cover Page | Oct. 29, 2019 |
| Cover page. | |
| Entity Central Index Key | 0000909108 |
| Title of 12(b) Security | Common Stock, no par value |
| Document Period End Date | Oct. 29, 2019 |
| Entity Registrant Name | DIAMOND HILL INVESTMENT GROUP INC |
| Entity Incorporation, State or Country Code | OH |
| Document Type | 8-K |
| Entity File Number | 000-24498 |
| Entity Tax Identification Number | 65-0190407 |
| Entity Address, Address Line One | 325 John H. McConnell Blvd, Suite 200 |
| Entity Address, City or Town | Columbus |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 43215 |
| City Area Code | 614 |
| Local Phone Number | 255-3333 |
| Trading Symbol | DHIL |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
