Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES:
Net income	$ 151,086	$ 142,667	$ 131,461
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	112,004	107,951	106,073
Regulatory debits	1,148	1,107	2,334
Regulatory charges resulting from tax rate changes	23,580	23,580	35,680
Amortization of deferred charges and credits	(3,526)	(2,806)	954
Allowance for equity funds used during construction (equity AFUDC)	(414)	(2,744)	(1,703)
Changes in current assets and liabilities:
Trade and other accounts receivable	3,606	(1,528)	(1,611)
Affiliated receivables	58	(91)	1,903
Materials and supplies	(124)	363	39
Other current assets	337	4,311	4,494
Trade accounts payable	5,140	(1,497)	1,753
Affiliated payables	(5,062)	(17,360)	15,234
Other accrued liabilities	(8,603)	(8,056)	881
Changes in noncurrent assets and liabilities:
Regulatory liabilities	4,185	3,493	2,530
Other, net	503	8,570	(4,894)
Net cash provided by operating activities	283,918	257,960	295,128
FINANCING ACTIVITIES:
Proceeds from long-term debt	0	0	246,395
Retirement of long-term debt	0	0	(250,000)
Payments for debt issuance costs	0	(59)	(2,498)
Cash distributions to parent	(164,000)	(110,000)	(174,000)
Net cash used in financing activities	(164,000)	(110,059)	(180,103)
Property, plant and equipment:
Capital expenditures	(78,497)	(128,440)	(73,487)
Contributions and advances for construction costs	6,103	4,805	2,075
Disposal of property, plant and equipment, net	(920)	(3,401)	(879)
Advances to affiliates, net	(46,604)	(20,865)	(42,734)
Net cash used in investing activities	(119,918)	(147,901)	(115,025)
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	(78,602)	(132,260)	(73,105)
Changes in related accounts receivable, accounts payable, and accrued liabilities	105	3,820	(382)
Capital expenditures	$ (78,497)	$ (128,440)	$ (73,487)