CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Cash Flows [Abstract]
Net Income	$ 124,438	$ 127,794	$ 102,338
Adjustments to Reconcile Net Income To Net Cash Flows from Operating Activities:
Depreciation Expense	146,097	138,093	126,520
Amortization Expense	22,498	19,261	28,567
Amortization of Debt Issuance Costs	2,853	3,043	2,626
Use of Renewable Energy Credits for Compliance	17,618	19,731	17,818
Deferred Income Taxes	59,367	72,026	59,024
Pension and Other Postretirement Benefits Expense	15,338	18,588	13,648
Pension and Other Postretirement Benefits Funding	(13,459)	(30,682)	(14,388)
Allowance for Equity Funds Used During Construction	(4,522)	(5,352)	(6,677)
Fortis Acquisition Direct Customer Benefit	0	0	18,870
FERC Transmission Refund Payable	4,878	0	0
Changes in Current Assets and Current Liabilities:
Accounts Receivable	7,809	(3,019)	(12,886)
Materials, Supplies, and Fuel Inventory	7,627	(8,758)	666
Regulatory Assets	(12,147)	18,002	(12,777)
Accounts Payable and Accrued Charges	14,284	(13,917)	(8,763)
Regulatory Liabilities	18,012	10,921	8,388
Other, Net	14,773	(797)	(9,311)
Net Cash Flows—Operating Activities	425,464	364,934	313,663
Cash Flows from Investing Activities
Capital Expenditures	(250,360)	(333,841)	(323,524)
Purchase, Springerville Coal Handling Facilities Lease Assets	0	(120,312)	0
Proceeds from Sale, Springerville Coal Handling Facilities	0	23,656	0
Purchase, Springerville Unit 1 Assets	(85,000)	(45,753)	(19,608)
Purchase, Gila River Unit 3	0	0	(163,938)
Purchase Intangibles, Renewable Energy Credits	(40,949)	(29,184)	(28,334)
Contributions in Aid of Construction	3,432	4,517	15,903
Other, Net	(3,176)	(1,974)	1,863
Net Cash Flows—Investing Activities	(376,053)	(502,891)	(517,638)
Cash Flows from Financing Activities
Proceeds from Borrowings, Revolving Credit Facilities	0	148,000	275,000
Repayments of Borrowings, Revolving Credit Facilities	0	(233,000)	(190,000)
Proceeds from Borrowings, Term Loan	0	130,000	0
Repayments of Borrowings, Term Loan	0	(130,000)	0
Proceeds from Issuance, Long-Term Debt	0	299,019	149,168
Repayments, Long-Term Debt	0	(208,600)	0
Dividends Paid to Parent	(50,000)	(50,000)	(40,000)
Payments of Capital Lease Obligations	(14,079)	(13,464)	(165,145)
Payment of Debt Issuance/Retirement Costs	(183)	(3,942)	(1,856)
Contribution from Parent	0	180,000	225,000
Other, Net	(4,871)	1,458	643
Net Cash Flows—Financing Activities	(69,133)	119,471	252,810
Net Increase (Decrease) in Cash and Cash Equivalents	(19,722)	(18,486)	48,835
Cash and Cash Equivalents, Beginning of Period	55,684	74,170	25,335
Cash and Cash Equivalents, End of Period	$ 35,962	$ 55,684	$ 74,170