v3.26.1
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Defined benefit plan, amortization of net loss (gain) $ (4,212) $ (1,834)  
Defined benefit plan, amortization of unrecognized prior service cost (credit) (93) (93)  
Defined benefit plan, reclassification adjustment, before tax and regulatory adjustments (4,305) (1,927)  
Defined benefit plan, regulatory adjustments 4,305 1,927  
Debt Securities, Available-for-Sale, Realized Gain (Loss) 32 3  
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax 32 3  
Reclassification from AOCI, Current Period, Tax (7) (1)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 25 2  
Accumulated other comprehensive income (loss), net of tax (179)   $ 4
ONE Gas Pension Plans [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Defined benefit plan, amortization of net loss (gain) (4,220) (1,865)  
Defined benefit plan, amortization of unrecognized prior service cost (credit) (93) (93)  
Defined benefit plan, reclassification from AOCI, Current Period, before Tax, Attributable to Parent $ 0 $ 0