CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash Flows from Operating Activities
Net Income	$ 201,245	$ 191,405	$ 186,515
Adjustments to Reconcile Net Income to Net Cash Flows from Operating Activities:
Depreciation Expense	201,524	189,051	169,042
Amortization Expense	43,995	28,754	27,706
Amortization of Debt Issuance Costs	2,829	2,721	2,326
Use of Renewable Energy Credits for Compliance	45,815	44,517	37,141
Deferred Income Taxes	37,217	39,408	41,614
Pension and Other Postretirement Benefits Expense	15,342	14,883	17,762
Pension and Other Postretirement Benefits Funding	(20,806)	(21,018)	(16,749)
Allowance for Equity Funds Used During Construction	(17,885)	(22,847)	(15,222)
Regulatory Deferral, ACC Refund Order	0	(7,705)	7,705
Changes in Current Assets and Current Liabilities:
Accounts Receivable	(18,738)	(19,019)	9,238
Materials, Supplies, and Fuel Inventory	(18,445)	(3,460)	(16,236)
Regulatory Assets	(59,542)	5,339	(20,934)
Other Current Assets	4,670	(8,311)	(475)
Accounts Payable and Accrued Charges	14,979	(20,885)	(27,776)
Income Taxes Payable	(3,271)	10,245	6,072
Regulatory Liabilities	(9,599)	41,287	(1,626)
Other, Net	8,724	1,689	8,140
Net Cash Flows—Operating Activities	428,054	466,054	414,243
Cash Flows from Investing Activities
Capital Expenditures	(499,405)	(839,958)	(607,593)
Proceeds from Sale, Springerville Common Facilities	0	29,569	0
Purchase Intangibles, Renewable Energy Credits	(55,297)	(53,509)	(51,699)
Purchase, Other Investments	0	(8,500)	0
Contributions in Aid of Construction	5,678	4,615	6,607
Note Receivable	0	0	(1,000)
Net Cash Flows—Investing Activities	(549,024)	(867,783)	(653,685)
Cash Flows from Financing Activities
Proceeds from Borrowings, Revolving Credit Facility	50,000	105,000	0
Repayments of Borrowings, Revolving Credit Facility	(35,000)	(105,000)	0
Proceeds from Borrowings, Term Loan	0	60,000	165,000
Repayments of Borrowings, Term Loan	0	(225,000)	0
Proceeds from Issuance, Long-Term Debt—Net of Discount	322,231	645,768	0
Repayments of Long-Term Debt	(250,000)	(180,410)	(14,700)
Dividends Paid to Parent	(62,500)	(75,000)	(75,000)
Payments of Finance Lease Obligations	0	(17,087)	(10,890)
Payment of Debt Issuance Costs	(4,382)	(6,327)	(757)
Contributions from Parent	50,000	250,000	50,000
Other	2,107	3,316	1,514
Net Cash Flows—Financing Activities	72,456	455,260	115,167
Net Increase (Decrease) in Cash, Cash Equivalents, and Restricted Cash	(48,514)	53,531	(124,275)
Cash, Cash Equivalents, and Restricted Cash, Beginning of Period	82,003	28,472	152,747
Cash, Cash Equivalents, and Restricted Cash, End of Period	$ 33,489	$ 82,003	$ 28,472