v3.26.1
EMPLOYEE BENEFIT PLANS (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Employee Benefit Plans [Line Items]          
Pension and other postemployment benefits $ 47,326   $ 47,326   $ 47,012
Defined benefit plan, amortization of unrecognized prior service cost (credit) 93 $ 93 186 $ 186  
Defined benefit plan, amortization of net loss (gain) 4,212 1,834 8,424 3,668  
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax (1) (1) (1) (1)  
Regulatory Assets 334,071   334,071   305,729
Other Deferred Compensation Arrangements, Liability, Current and Noncurrent 24,600   24,600   21,900
Cash Surrender Value of Life Insurance 49,200   49,200   46,200
Deferred Compensation Arrangement with Individual, Compensation Expense     2,100 1,100  
Regulatory liability, noncurrent 433,135   433,135   451,620
Regulatory Deferral of net periodic benefit costs     (2,200) (500)  
Capitalized non-service cost components (5,100)   (5,100)   $ (6,800)
ONE Gas Pension Plans [Member]          
Employee Benefit Plans [Line Items]          
Service cost 1,291 1,337 2,582 2,674  
Interest cost 9,525 10,042 19,050 20,084  
Expected return on assets (10,489) (11,910) (20,979) (23,820)  
Defined benefit plan, amortization of unrecognized prior service cost (credit) 93 93 186 186  
Defined benefit plan, amortization of net loss (gain) 4,220 1,865 8,440 3,730  
Net periodic benefit cost (credit) 4,640 1,427 9,279 2,854  
ONE Gas Postretirement Benefit Plans [Member]          
Employee Benefit Plans [Line Items]          
Service cost 112 123 222 246  
Interest cost 2,020 2,069 4,040 4,138  
Expected return on assets (2,194) (2,212) (4,388) (4,424)  
Defined benefit plan, amortization of net loss (gain) (8) (31) (16) (62)  
Net periodic benefit cost (credit) $ (70) $ (51) $ (142) $ (102)