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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

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