Document And Entity Information - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2019
Feb. 07, 2020
Jun. 30, 2019
Document Information [Line Items]      
Entity File Number 001-36108    
Entity Registrant Name ONE Gas, Inc.    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Entity Incorporation, State or Country Code OK    
Entity Central Index Key 0001587732    
Current Fiscal Year End Date --12-31    
Entity Common Stock, Shares Outstanding   52,774,254  
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Document Type 10-K    
Document Annual Report true    
Amendment Flag false    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity Tax Identification Number 46-3561936    
Entity Address, Address Line One 15 East Fifth Street    
Entity Address, City or Town Tulsa,    
Entity Address, State or Province OK    
Entity Address, Postal Zip Code 74103    
City Area Code (918)    
Local Phone Number 947-7000    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Trading Symbol OGS    
Security Exchange Name NYSE    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 4.5
Documents Incorporated by Reference [Text Block] Portions of the definitive proxy statement to be delivered to shareholders in connection with the Annual Meeting of Shareholders to be held May 21, 2020, are incorporated by reference in Part III.