EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	$ (1,149)	$ (908)	$ (2,298)	$ (1,816)
Amortization of net loss	10,648	10,036	21,296	20,073
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,044	3,014	6,088	6,028
Interest cost	10,113	11,388	20,226	22,775
Expected return on assets	(14,624)	(15,296)	(29,248)	(30,592)
Amortization of net loss	9,027	8,885	18,054	17,771
Net periodic benefit cost	7,560	7,991	15,120	15,982
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	627	638	1,254	1,276
Interest cost	2,472	2,627	4,944	5,254
Expected return on assets	(3,147)	(3,071)	(6,294)	(6,142)
Amortization of unrecognized prior service cost	(1,149)	(908)	(2,298)	(1,816)
Amortization of net loss	1,621	1,151	3,242	2,302
Net periodic benefit cost	$ 424	$ 437	$ 848	$ 874