EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Employee Benefit Plans [Line Items]
Document Period End Date	Mar. 31, 2020
Capitalized non-service cost components as a regulatory asset	$ 5,400		$ 4,700
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	29	$ 168
Amortization of net loss	(10,623)	(8,821)
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,217	3,008
Interest cost	8,545	10,168
Expected return on assets	(15,280)	(15,485)
Amortization of net loss	10,580	8,260
Net periodic benefit cost	7,062	5,951
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	423	434
Interest cost	1,889	2,329
Expected return on assets	(3,867)	(3,147)
Amortization of unrecognized prior service cost	(29)	(168)
Amortization of net loss	43	561
Net periodic benefit cost	$ (1,541)	$ 9