EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Employee Benefit Plans [Line Items]
Capitalized non-service cost components as a regulatory asset	$ 2,900		$ 2,900		$ 1,500
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	168	$ 1,142	336	$ 2,284
Amortization of net loss	(8,821)	(10,950)	(17,642)	(21,900)
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,008	3,230	6,016	6,460
Interest cost	10,168	9,200	20,336	18,400
Expected return on assets	(15,485)	(15,145)	(30,970)	(30,290)
Amortization of net loss	8,260	9,978	16,520	19,956
Net periodic benefit cost	5,951	7,263	11,902	14,526
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	434	589	868	1,178
Interest cost	2,329	2,279	4,658	4,558
Expected return on assets	(3,147)	(3,571)	(6,294)	(7,142)
Amortization of unrecognized prior service cost	(168)	(1,142)	(336)	(2,284)
Amortization of net loss	561	972	1,122	1,944
Net periodic benefit cost	$ 9	$ (873)	$ 18	$ (1,746)