EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Benefit Plans [Line Items]
Defined Benefit Plan, Settlements, Plan Assets	$ 47,000
Defined Benefit Plan, Settlements, Benefit Obligation	47,000
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	(1,149)	$ (909)	$ (3,447)	$ (2,725)
Amortization of net loss	10,648	10,040	31,944	30,113
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,044	3,014	9,132	9,042
Interest cost	10,113	11,387	30,339	34,162
Expected return on assets	(14,624)	(15,296)	(43,872)	(45,888)
Amortization of net loss	9,027	8,886	27,081	26,657
Net periodic benefit cost	7,560	7,991	22,680	23,973
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	627	637	1,881	1,913
Interest cost	2,472	2,626	7,416	7,880
Expected return on assets	(3,147)	(3,070)	(9,441)	(9,212)
Amortization of unrecognized prior service cost	(1,149)	(909)	(3,447)	(2,725)
Amortization of net loss	1,621	1,154	4,863	3,456
Net periodic benefit cost	$ 424	$ 438	$ 1,272	$ 1,312