CREDIT FACILITIES (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Short-term Debt [Line Items]
Line of Credit Facility Sublimit	$ 20,000
Line of Credit Facility Option to Increase Borrowing Capacity	$ 500,000
Ratio of Indebtedness to Net Capital	0.45
Letters of Credit Outstanding, Amount	$ 1,200
Line of Credit Facility, Remaining Borrowing Capacity	698,800
Commercial paper maximum borrowing capacity	$ 700,000		$ 700,000
Document Period End Date	Mar. 31, 2020
Commercial Paper	$ 474,695	$ 516,500
Proceeds from Unsecured Lines of Credit	$ 250,000
Line of Credit Facility, Description	In April 2020, we entered into the ONE Gas 364-day Credit Agreement. The ONE Gas 364-day Credit Agreement is a $250 million revolving unsecured credit facility containing various customary conditions to borrowing and affirmative, negative and financial ratio maintenance covenants, all of which are substantially the same as those of the ONE Gas Credit Agreement. The ONE Gas 364-day Credit Agreement also contains provisions for an applicable margin rate and a quarterly facility fee, both of which adjust with changes in our credit rating.  Based on our current credit ratings, borrowings, if any, will accrue interest at LIBOR plus 115 basis points, and the quarterly facility fee is 10 basis points. In the event LIBOR is not available, and such circumstances are unlikely to be temporary, our lenders may establish an alternative interest rate for the impacted loans by replacing LIBOR with one or more secured overnight financing-based rates or another alternate benchmark rate. We have not borrowed on the ONE Gas 364-day Credit Agreement.
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.