EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Employee Benefit Plans [Line Items]
Document Period End Date			Jun. 30, 2020
Capitalized non-service cost components as a regulatory asset	$ 5,900		$ 5,900		$ 4,700
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	29	$ 168	58	$ 336
Amortization of net loss	(10,623)	(8,821)	(21,246)	(17,642)
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,217	3,008	6,434	6,016
Interest cost	8,545	10,168	17,090	20,336
Expected return on assets	(15,280)	(15,485)	(30,560)	(30,970)
Amortization of net loss	10,580	8,260	21,160	16,520
Net periodic benefit cost	7,062	5,951	14,124	11,902
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	423	434	846	868
Interest cost	1,889	2,329	3,778	4,658
Expected return on assets	(3,867)	(3,147)	(7,734)	(6,294)
Amortization of unrecognized prior service cost	(29)	(168)	(58)	(336)
Amortization of net loss	43	561	86	1,122
Net periodic benefit cost	$ (1,541)	$ 9	$ (3,082)	$ 18