Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
OPERATING ACTIVITIES:
Net income	$ 179,719	$ 136,774	$ 135,508
Adjustments to reconcile net cash provided (used) by operating activities:
Depreciation and amortization	111,434	114,384	113,180
Regulatory charges resulting from tax rate changes	0	19,621	23,580
Allowance for equity funds used during construction (equity AFUDC)	(3,155)	(2,335)	(1,360)
Changes in current assets and liabilities:
Trade and other accounts receivable	(213)	(515)	(1,632)
Affiliate receivables	454	(1,457)	(321)
Materials and supplies	1,166	(16)	898
Regulatory assets	(340)	(445)	111
Other current assets	1,432	(345)	(221)
Trade accounts payable	4,295	(1,230)	(3,571)
Affiliate payables	665	(1,025)	9,030
Regulatory liabilities	(125,502)	145,242	2
Other accrued liabilities	3,021	1,361	(877)
Regulatory assets	(28,813)	0	0
Regulatory liabilities	(19,530)	(140,190)	4,374
Other, net	4,615	(1,182)	(5,739)
Net cash provided (used) by operating activities	129,248	268,642	272,962
FINANCING ACTIVITIES:
Cash distributions to parent	(154,800)	(137,900)	(166,500)
Net cash provided (used) by financing activities	(154,800)	(137,900)	(166,500)
Property, plant and equipment:
Capital expenditures	(124,932)	(106,729)	(78,787)
Contributions and advances for construction costs	(6,066)	(10,555)	(10,268)
Disposal of property, plant and equipment, net	(8,447)	(5,369)	(4,370)
Advances to affiliate, net	152,865	(29,199)	(33,573)
Net cash provided (used) in investing activities	25,552	(130,742)	(106,462)
Increase (decrease) in cash	0	0	0
Cash at beginning of period	0	0	0
Supplemental Cash Flow Elements [Abstract]
Increases to property, plant and equipment, exclusive of equity AFUDC	(139,620)	(109,359)	(88,791)
Changes in related accounts payable and accrued liabilities	14,688	2,630	10,004
Capital expenditures	(124,932)	(106,729)	(78,787)
Cash at end of period	$ 0	$ 0	$ 0