EMPLOYEE BENEFIT PLANS (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Employee Benefit Plans [Line Items]
Document Period End Date			Sep. 30, 2020
Capitalized non-service cost components as a regulatory asset	$ 6,600,000		$ 6,600,000		$ 4,700,000
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	29,000	$ 168,000	87,000	$ 504,000
Amortization of net loss	(10,623,000)	(8,821,000)	(31,869,000)	(26,463,000)
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,217,000	3,008,000	9,651,000	9,024,000
Interest cost	8,545,000	10,168,000	25,635,000	30,504,000
Expected return on assets	(15,280,000)	(15,485,000)	(45,840,000)	(46,455,000)
Amortization of net loss	10,580,000	8,260,000	31,740,000	24,780,000
Net periodic benefit cost	7,062,000	5,951,000	21,186,000	17,853,000
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	423,000	434,000	1,269,000	1,302,000
Interest cost	1,889,000	2,329,000	5,667,000	6,987,000
Expected return on assets	(3,867,000)	(3,147,000)	(11,601,000)	(9,441,000)
Amortization of unrecognized prior service cost	(29,000)	(168,000)	(87,000)	(504,000)
Amortization of net loss	43,000	561,000	129,000	1,683,000
Net periodic benefit cost	$ (1,541,000)	$ 9,000	$ (4,623,000)	$ 27,000