EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	$ (1,149)	$ (908)
Amortization of net loss	10,648	10,037
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,044	3,014
Interest cost	10,113	11,387
Expected return on assets	(14,624)	(15,296)
Amortization of net loss	9,027	8,886
Net periodic benefit cost	7,560	7,991
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	627	638
Interest cost	2,472	2,627
Expected return on assets	(3,147)	(3,071)
Amortization of unrecognized prior service cost	(1,149)	(908)
Amortization of net loss	1,621	1,151
Net periodic benefit cost	$ 424	$ 437