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EMPLOYEE BENEFIT PLANS (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Components of net periodic benefit cost:        
Amortization of unrecognized prior service cost $ 1,142 $ 1,149 $ 2,284 $ 2,298
Amortization of net loss (10,950) (10,648) (21,900) (21,296)
ONE Gas Pension Plans [Member]        
Components of net periodic benefit cost:        
Service cost 3,230 3,044 6,460 6,088
Interest cost 9,200 10,113 18,400 20,226
Expected return on assets (15,145) (14,624) (30,290) (29,248)
Amortization of net loss 9,978 9,027 19,956 18,054
Net periodic benefit cost 7,263 7,560 14,526 15,120
ONE Gas Postretirement Benefit Plans [Member]        
Components of net periodic benefit cost:        
Service cost 589 627 1,178 1,254
Interest cost 2,279 2,472 4,558 4,944
Expected return on assets (3,571) (3,147) (7,142) (6,294)
Amortization of unrecognized prior service cost (1,142) (1,149) (2,284) (2,298)
Amortization of net loss 972 1,621 1,944 3,242
Net periodic benefit cost $ (873) $ 424 $ (1,746) $ 848