Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
OPERATING ACTIVITIES:
Net income	$ 135,508	$ 151,086	$ 142,667
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	113,180	112,004	107,951
Regulatory debits	1,436	1,148	1,107
Regulatory charges resulting from tax rate changes	23,580	23,580	23,580
Amortization of deferred charges and credits	(4,414)	(3,526)	(2,806)
Allowance for equity funds used during construction (equity AFUDC)	(1,360)	(414)	(2,744)
Changes in current assets and liabilities:
Trade and other accounts receivable	(1,632)	3,606	(1,528)
Affiliated receivables	(321)	58	(91)
Materials and supplies	898	(124)	363
Other current assets	(110)	337	4,311
Trade accounts payable	(3,571)	5,140	(1,497)
Affiliated payables	9,030	(5,062)	(17,360)
Other accrued liabilities	(875)	(8,603)	(8,056)
Changes in noncurrent assets and liabilities:
Regulatory liabilities	4,374	4,185	3,493
Other, net	(2,761)	503	8,570
Net cash provided by operating activities	272,962	283,918	257,960
FINANCING ACTIVITIES:
Payments of Debt Issuance Costs	0	0	59
Cash distributions to parent	(166,500)	(164,000)	(110,000)
Net cash used in financing activities	(166,500)	(164,000)	(110,059)
Property, plant and equipment:
Capital expenditures	(78,787)	(78,497)	(128,440)
Contributions and advances for construction costs	10,268	6,103	4,805
Disposal of property, plant and equipment, net	(4,370)	(920)	(3,401)
Advances to affiliates, net	(33,573)	(46,604)	(20,865)
Net cash used in investing activities	(106,462)	(119,918)	(147,901)
NET (DECREASE) INCREASE IN CASH	0	0	0
CASH AT BEGINNING OF PERIOD	0	0	0
CASH AT END OF PERIOD	0	0	0
Supplemental Cash Flow Elements [Abstract]
Increases to property, plant and equipment, exclusive of equity AFUDC	(88,791)	(78,602)	(132,260)
Changes in related accounts receivable, accounts payable, and accrued liabilities	10,004	105	3,820
Capital expenditures	$ (78,787)	$ (78,497)	$ (128,440)