ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]
Defined benefit plan, amortization of net gain (loss)	$ (1,834)	$ (1,442)	$ (5,502)	$ (4,326)
Defined benefit plan, amortization of unrecognized prior service (credit) cost	(93)	(93)	(279)	(279)
Defined benefit plan, reclassification adjustment, before tax and regulatory adjustments	(1,927)	(1,535)	(5,781)	(4,605)
Defined benefit plan, regulatory adjustments	1,928	1,536	5,783	4,607
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	0	(1)	7	(11)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	1	0	9	(9)
Reclassification from AOCI, Current Period, Tax	0	0	(2)	2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1	0	7	(7)
ONE Gas Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Defined benefit plan, amortization of net gain (loss)	(1,865)	(1,446)	(5,595)	(4,338)
Defined benefit plan, amortization of unrecognized prior service (credit) cost	(93)	(93)	(279)	(279)
Defined benefit plan, reclassification from AOCI, Current Period, before Tax, Attributable to Parent	$ 1	$ 1	$ 2	$ 2