EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Employee Benefit Plans [Line Items]
Capitalized non-service cost components as a regulatory asset	$ 2,200		$ 1,500
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	168	$ 1,142
Amortization of net loss	(8,821)	(10,950)
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,008	3,230
Interest cost	10,168	9,200
Expected return on assets	(15,485)	(15,145)
Amortization of net loss	8,260	9,978
Net periodic benefit cost	5,951	7,263
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	434	589
Interest cost	2,329	2,279
Expected return on assets	(3,147)	(3,571)
Amortization of unrecognized prior service cost	(168)	(1,142)
Amortization of net loss	561	972
Net periodic benefit cost	$ 9	$ (873)