EMPLOYEE BENEFIT PLANS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost	$ (373)	$ (303)
Amortization of net loss	12,565	8,542
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	3,524	2,768
Interest cost	10,652	10,948
Expected return on assets	(15,362)	(14,965)
Amortization of unrecognized prior service cost	67	137
Amortization of net loss	11,055	7,550
Net periodic benefit cost	9,936	6,438
ONE Gas Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	849	1,174
Interest cost	2,666	2,901
Expected return on assets	(2,908)	(2,848)
Amortization of unrecognized prior service cost	(440)	(440)
Amortization of net loss	1,510	992
Net periodic benefit cost	$ 1,677	$ 1,779