CREDIT FACILITIES (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Short-term Debt [Line Items]
Line of Credit Facility Sublimit	$ 20,000
Line of Credit Facility Option to Increase Borrowing Capacity	$ 500,000
Line of Credit Facility, Expiration Date	Oct. 01, 2023
Ratio of Indebtedness to Net Capital	0.43
Letters of Credit Outstanding, Amount	$ 1,200
Line of Credit Facility, Remaining Borrowing Capacity	698,800
Commercial paper maximum borrowing capacity	700,000
Commercial Paper	$ 295,500	$ 299,500
Line of Credit [Member]
Short-term Debt [Line Items]
Debt Instrument, Covenant Description	The ONE Gas Credit Agreement contains certain financial, operational and legal covenants. Among other things, these covenants include maintaining ONE Gas’ total debt-to-capital ratio of no more than 70 percent at the end of any calendar quarter.