EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Employee Benefit Plans [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.75%		3.75%		4.75%
Components of net periodic benefit cost:
Amortization of unrecognized prior service cost		$ (909)	$ (374)	$ (2,725)	$ (1,120)
Amortization of net loss		10,040	12,564	30,113	37,694
Defined Benefit Plan, Effect of Plan Amendment on Net Periodic Benefit Cost		$ 800		$ 800
Discount Rate		4.75%		4.75%
Expected long-term return on plan assets				8.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				7.75%		8.00%
Subsequent Event [Member]
Components of net periodic benefit cost:
Discount Rate	3.75%
Expected long-term return on plan assets	7.75%
ONE Gas Pension Plans [Member]
Components of net periodic benefit cost:
Service cost		$ 3,014	3,497	$ 9,042	10,518
Interest cost		11,387	10,652	34,162	31,956
Expected return on assets		(15,296)	(15,363)	(45,888)	(46,087)
Amortization of unrecognized prior service cost		0	66	0	200
Amortization of net loss		8,886	11,054	26,657	33,164
Net periodic benefit cost		7,991	9,906	23,973	29,751
ONE Gas Postretirement Benefit Plans [Member]
Employee Benefit Plans [Line Items]
Defined Benefit Plan, Plan Amendments				31,500
Components of net periodic benefit cost:
Service cost		637	849	1,913	2,547
Interest cost		2,626	2,665	7,880	7,997
Expected return on assets		(3,070)	(2,908)	(9,212)	(8,724)
Amortization of unrecognized prior service cost		(909)	(440)	(2,725)	(1,320)
Amortization of net loss		1,154	1,510	3,456	4,530
Net periodic benefit cost		$ 438	$ 1,676	$ 1,312	$ 5,030