Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Mar. 02, 2021	Jun. 30, 2020
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	0-18082
Entity Registrant Name	GREAT SOUTHERN BANCORP, INC.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	43-1524856
Entity Address, Address Line One	1451 E. 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
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	GSBC
Trading Exchange	NASDAQ
Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Catagory	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Number of common stock shares outstanding		13,690,119
Entity Public Float			$ 438,764,780
Entity Central Index Key	0000854560
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false