| Document and Entity Information | Sep. 16, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 16, 2020 |
| Entity Registrant Name | GREAT SOUTHERN BANCORP, INC. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 000-18082 |
| Entity Tax Identification Number | 43-1524856 |
| Entity Address, Address Line One | 1451 East Battlefield |
| Entity Address, City or Town | Springfield |
| Entity Address, State or Province | MO |
| Entity Address, Postal Zip Code | 65804 |
| City Area Code | 417 |
| Local Phone Number | 887-4400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000854560 |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | GSBC |
| Security Exchange Name | NASDAQ |
