v3.26.1
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Defined benefit plan, amortization of net loss (gain) $ (4,212) $ (1,834) $ (8,424) $ (3,668)
Defined benefit plan, amortization of unrecognized prior service cost (credit) (93) (93) (186) (186)
Defined benefit plan, reclassification adjustment, before tax and regulatory adjustments (4,305) (1,927) (8,610) (3,854)
Defined benefit plan, regulatory adjustments 4,306 1,928 8,611 3,855
Debt Securities, Available-for-Sale, Realized Gain (Loss) 3 4 35 7
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax 4 5 36 8
Reclassification from AOCI, Current Period, Tax (1) (1) (8) (2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 3 4 28 6
ONE Gas Pension Plans [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Defined benefit plan, amortization of net loss (gain) (4,220) (1,865) (8,440) (3,730)
Defined benefit plan, amortization of unrecognized prior service cost (credit) (93) (93) (186) (186)
Defined benefit plan, reclassification from AOCI, Current Period, before Tax, Attributable to Parent $ 1 $ 1 $ 1 $ 1