EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Employee Benefit Plans [Line Items]
Capitalized non-service cost components as a regulatory asset	$ 3,800		$ 3,800		$ 1,500
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	168	$ 1,142	504	$ 3,426
Amortization of net loss	(8,821)	(10,950)	(26,463)	(32,850)
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,008	3,230	9,024	9,690
Interest cost	10,168	9,200	30,504	27,600
Expected return on assets	(15,485)	(15,145)	(46,455)	(45,435)
Amortization of net loss	8,260	9,978	24,780	29,934
Net periodic benefit cost	5,951	7,263	17,853	21,789
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	434	589	1,302	1,767
Interest cost	2,329	2,279	6,987	6,837
Expected return on assets	(3,147)	(3,571)	(9,441)	(10,713)
Amortization of unrecognized prior service cost	(168)	(1,142)	(504)	(3,426)
Amortization of net loss	561	972	1,683	2,916
Net periodic benefit cost	$ 9	$ (873)	$ 27	$ (2,619)