v3.26.1
EMPLOYEE BENEFIT PLANS (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Employee Benefit Plans [Line Items]      
Pension and other postemployment benefits $ 47,175   $ 47,012
Defined benefit plan, amortization of unrecognized prior service cost (credit) 93 $ 93  
Defined benefit plan, amortization of net loss (gain) 4,212 1,834  
Regulatory Assets 313,535   305,729
Other Deferred Compensation Arrangements, Liability, Current and Noncurrent 21,900    
Cash Surrender Value of Life Insurance 45,400   46,200
Deferred Compensation Arrangement with Individual, Compensation Expense (500) (300)  
Regulatory liability, noncurrent 441,041   451,620
Regulatory Deferral of net periodic benefit costs (1,100) (300)  
Capitalized non-service cost components (6,000)   $ (6,800)
ONE Gas Pension Plans [Member]      
Employee Benefit Plans [Line Items]      
Service cost 1,291 1,337  
Interest cost 9,525 10,042  
Expected return on assets (10,490) (11,910)  
Defined benefit plan, amortization of unrecognized prior service cost (credit) 93 93  
Defined benefit plan, amortization of net loss (gain) 4,220 1,865  
Net periodic benefit cost (credit) 4,639 1,427  
ONE Gas Postretirement Benefit Plans [Member]      
Employee Benefit Plans [Line Items]      
Service cost 110 123  
Interest cost 2,020 2,069  
Expected return on assets (2,194) (2,212)  
Defined benefit plan, amortization of unrecognized prior service cost (credit) 0 0  
Defined benefit plan, amortization of net loss (gain) (8) (31)  
Net periodic benefit cost (credit) $ (72) $ (51)