Long-Term Debt and Financing Obligations Outstanding Balance (Details) $ in Thousands														1 Months Ended	12 Months Ended
		Aug. 15, 2017 USD ($)	Jan. 10, 2017 USD ($)	Jan. 09, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 24, 2016 USD ($)	Aug. 15, 2015 USD ($)	Dec. 01, 2014 USD ($)	Dec. 06, 2012 USD ($)	Aug. 31, 2010 USD ($)	Aug. 18, 2010 USD ($)	Jun. 02, 2008	May 02, 2005	Jun. 29, 2008 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017	Sep. 01, 2017 USD ($)	Aug. 31, 2017 USD ($)	Aug. 28, 2012	Jun. 01, 2008 USD ($)	May 01, 2005 USD ($)
Debt Instrument [Line Items]
Long-term Debt					$ 1,360,230										$ 1,369,521	$ 1,360,230
Long-term Debt, Current Maturities					83,143										0	83,143
Line of Credit, Current					(81,574)										(173,533)	(81,574)
Long-term debt, net of current portion					1,195,513										1,195,988	1,195,513
Line of Credit Facility, Expiration Date			Jan. 14, 2021
Line of Credit renewal term, year			1 year
Proceeds from issuance of senior notes															0	157,052	$ 0
Interest accrued					13,287										12,470	13,287
Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit					37,600										88,500	37,600
Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit					$ 44,000										$ 85,000	44,000
Short Term Borrowing
Debt Instrument [Line Items]
Repayments of Short-term Debt														$ 44,000
Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Weighted Average Interest Rate															2.70%
Revolving Credit Facility | Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit															$ 88,500
Revolving Credit Facility | Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit															85,000
Revolving Credit Facility Expiring January 2019
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date					Jan. 14, 2019
Line of Credit Facility, Current Borrowing Capacity					$ 300,000											300,000
Line of Credit Facility, Remaining Borrowing Capacity					100,000											100,000
Revolving Credit Facility Expiring January 2020
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date				Jan. 14, 2020
Line of Credit renewal term, year				1 year
Line of Credit Facility, Current Borrowing Capacity				$ 350,000
Line of Credit Facility, Remaining Borrowing Capacity			$ 50,000
Line of Credit Facility, Maximum Borrowing Capacity			$ 400,000
Line of Credit Facility, Increase (Decrease), Net				$ 50,000
Bonds and Notes
Debt Instrument [Line Items]
Long-term Debt					1,278,656										1,195,988	1,278,656
Pollution Control Bonds
Debt Instrument [Line Items]
Long-term Debt	[1]				190,775										157,676	190,775
Long-term Debt, subject for redemption															$ 100,600
Number of tax exempt bond series															4
Debt Instrument, Face Amount															$ 159,800					$ 193,100
Pollution Control Bonds | 7.25% 2009 Series A Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[1]				$ 62,619										$ 62,657	$ 62,619
Debt Instrument, Interest Rate, Stated Percentage					7.25%										7.25%	7.25%
Debt Instrument, Interest Rate, Effective Percentage					7.46%										7.46%	7.46%
Debt Instrument, Maturity Date															Feb. 01, 2040	Feb. 01, 2040
Pollution Control Bonds | 4.50% 2012 Series A Refunding Bonds, due 2042
Debt Instrument [Line Items]
Long-term Debt	[1]				$ 58,471										$ 58,501	$ 58,471
Debt Instrument, Interest Rate, Stated Percentage					4.50%										4.50%	4.50%
Debt Instrument, Interest Rate, Effective Percentage					4.63%										4.63%	4.63%
Debt Instrument, Maturity Date															Aug. 01, 2042	Aug. 01, 2042
Pollution Control Bonds | 7.25% 2009 Series B Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[1]				$ 36,492										$ 36,518	$ 36,492
Debt Instrument, Interest Rate, Stated Percentage					7.25%										7.25%	7.25%
Debt Instrument, Interest Rate, Effective Percentage					7.49%										7.49%	7.49%
Debt Instrument, Maturity Date															Apr. 01, 2040	Apr. 01, 2040
Pollution Control Bonds | 1.875% 2012 Series A Refunding Bonds, due 2032
Debt Instrument [Line Items]
Long-term Debt	[1]				$ 33,193										$ 0	$ 33,193
Long-term Debt, Current Maturities																			$ 33,300
Debt Instrument, Interest Rate, Stated Percentage					1.875%										1.875%	1.875%		1.875%			1.875%
Debt Instrument, Interest Rate, Effective Percentage					2.35%										2.35%	2.35%
Debt Instrument, Maturity Date															Jun. 01, 2032	Jun. 01, 2032
Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[2]				$ 993,086										$ 993,426	$ 993,086
Senior Notes | 6.00% Senior Notes, Net of Discount, due 2035
Debt Instrument [Line Items]
Long-term Debt	[2]				$ 393,861										$ 394,040	$ 393,861
Debt Instrument, Interest Rate, Stated Percentage					6.00%										6.00%	6.00%							6.00%
Debt Instrument, Interest Rate, Effective Percentage					6.58%										6.58%	6.58%
Debt Instrument, Maturity Date															May 15, 2035	May 15, 2035
Debt Instrument, Face Amount																							$ 400,000
Debt Instrument, Issuance Date													May 01, 2005
Debt Instrument, Unamortized Discount																							$ 2,300
Senior Notes | 7.50% Senior Notes, Net of Discount, due 2038
Debt Instrument [Line Items]
Long-term Debt	[2]				$ 147,331										$ 147,384	$ 147,331
Debt Instrument, Interest Rate, Stated Percentage					7.50%										7.50%	7.50%						7.50%
Debt Instrument, Interest Rate, Effective Percentage					7.67%										7.67%	7.67%
Debt Instrument, Maturity Date															Mar. 15, 2038	Mar. 15, 2038
Debt Instrument, Face Amount																						$ 150,000
Debt Instrument, Issuance Date												Jun. 01, 2008
Debt Instrument, Unamortized Discount																						$ 1,300
Senior Notes | 3.30% Senior Notes, Net of Discount, due 2022
Debt Instrument [Line Items]
Long-term Debt	[2]				$ 148,939										$ 149,101	$ 148,939
Debt Instrument, Interest Rate, Stated Percentage					3.30%				3.30%						3.30%	3.30%
Debt Instrument, Interest Rate, Effective Percentage					3.43%										3.43%	3.43%
Debt Instrument, Maturity Date															Dec. 15, 2022	Dec. 15, 2022
Debt Instrument, Face Amount									$ 150,000
Debt Instrument, Issuance Date									Dec. 01, 2012
Debt Instrument, Unamortized Discount									$ 300
Senior Notes | 5.00% Senior Notes, Net of Discount, due 2044
Debt Instrument [Line Items]
Long-term Debt	[2]				$ 302,955										$ 302,901	$ 302,955
Debt Instrument, Interest Rate, Stated Percentage					5.00%	5.00%									5.00%	5.00%
Debt Instrument, Interest Rate, Effective Percentage					4.93%										4.93%	4.93%
Debt Instrument, Maturity Date						Dec. 01, 2044									Dec. 01, 2044	Dec. 01, 2044
Debt Instrument, Face Amount						$ 150,000		$ 150,000							$ 300,000
Proceeds from issuance of senior notes						158,100
Interest accrued						2,400
Debt Instrument, Unamortized Premium						$ 7,100
Debt Instrument, Issuance Date						Mar. 24, 2016		Dec. 01, 2014
Debt Instrument, Unamortized Discount								$ 500
RGRT Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[3]				$ 94,795										44,886	$ 94,795
Debt Instrument, Face Amount											$ 110,000
Redemption Price of Notes Equal to Principal Amount, Percentage											100.00%
Proceeds from Issuance of Long-term Debt										$ 109,400
RGRT Senior Notes | 3.67% Senior Notes, Series A, due 2015
Debt Instrument [Line Items]
Long-term Debt, Current Maturities							$ 15,000
Debt Instrument, Interest Rate, Stated Percentage							3.67%
Debt Instrument, Maturity Date							Aug. 15, 2015
RGRT Senior Notes | 4.47% Senior Notes, Series B, due 2017
Debt Instrument [Line Items]
Long-term Debt	[3]				$ 49,950										$ 0	$ 49,950
Long-term Debt, Current Maturities		$ 50,000
Debt Instrument, Interest Rate, Stated Percentage		4.47%			4.47%										4.47%	4.47%				4.47%
Debt Instrument, Interest Rate, Effective Percentage					4.62%										4.62%	4.62%
Debt Instrument, Maturity Date		Aug. 15, 2017													Aug. 15, 2017	Aug. 15, 2017
RGRT Senior Notes | 5.04% Senior Notes, Series C, due 2020
Debt Instrument [Line Items]
Long-term Debt	[3]				$ 44,845										$ 44,886	$ 44,845
Debt Instrument, Interest Rate, Stated Percentage					5.04%										5.04%	5.04%
Debt Instrument, Interest Rate, Effective Percentage					5.16%										5.16%	5.16%
Debt Instrument, Maturity Date															Aug. 15, 2020	Aug. 15, 2020
Financing Obligations | Revolving Credit Facility
Debt Instrument [Line Items]
Long-term Line of Credit	[4]				$ 81,574										$ 173,533	$ 81,574

[1] Pollution Control Bonds ("PCBs") The Company had four series of tax exempt unsecured PCBs in aggregate principal amount of $193.1 million. In September 2017, the $33.3 million 2012 Series A 1.875% PCBs, which were subject to mandatory tender for purchase, were redeemed and retired utilizing funds borrowed under the RCF. As of December 31, 2017, the Company's aggregate principal amount on PCBs was $159.8 million. The 7.25% 2009 Series A and the 7.25% 2009 Series B PCBs with an aggregate principal amount, together, of $100.6 million have optional redemptions beginning in February 2019 and April 2019, respectively.

[2] Senior Notes The Senior Notes are unsecured obligations of the Company. They were issued pursuant to bond covenants that provide limitations on the Company’s ability to enter into certain transactions. The 6.00% Senior Notes have an aggregate principal amount of $400.0 million and were issued in May 2005. The proceeds, net of a $2.3 million discount, were used to fund the retirement of the Company's first mortgage bonds. The Company amortizes the loss associated with a cash flow hedge recorded in accumulated other comprehensive income to earnings as interest expense over the life of the 6.00% Senior Notes. See Part II, Item 8, Financial Statements and Supplementary Data, Note O. This amortization is included in the effective interest rate of the 6.00% Senior Notes. The 7.50% Senior Notes have an aggregate principal amount of $150.0 million and were issued in June 2008. The proceeds, net of a $1.3 million discount, were used to repay short-term borrowings of $44.0 million, fund capital expenditures and for other general corporate purposes.The 3.30% Senior Notes have an aggregate principal amount of $150.0 million were issued in December 2012. The proceeds, net of a $0.3 million discount, were used to fund construction expenditures and for working capital and general corporate purposes. In December 2014, the Company issued 5.00% Senior Notes with an aggregate principal amount of $150.0 million. The proceeds, net of a $0.5 million discount, were used to fund construction expenditures and for working capital and general corporate purposes. In March 2016, the Company issued additional 5.00% Senior Notes with an aggregate principal amount of $150.0 million. The proceeds from this issuance, after deducting the underwriters' commission, were $158.1 million. These proceeds included accrued interest of $2.4 million and a $7.1 million premium before expenses. The net proceeds, from the sale of these senior notes were used to repay outstanding short-term borrowings under the RCF. After the March 2016 issuance, the Company's 5.00% Senior Notes due 2044 had a total principal amount outstanding of $300.0 million.

[3] RGRT Senior Notes In 2010, the Company and RGRT, a Texas grantor trust through which the Company finances its portion of fuel for Palo Verde, entered into a note purchase agreement with various institutional purchasers. Under the terms of the agreement, RGRT sold to the purchasers $110 million aggregate principal amount of Senior Notes ("RGRT Notes"). In August 2015 and 2017, $15.0 million and $50.0 million of these RGRT Notes, respectively, matured and were paid with borrowings from the RCF. The Company guarantees the payment of principal and interest on the RGRT Notes. In the Company’s financial statements, the assets and liabilities of RGRT are reported as assets and liabilities of the Company. RGRT pays interest on the RGRT Notes on February 15, and August 15 of each year until maturity. RGRT may redeem the RGRT Notes, in whole or in part, at any time at a redemption price equal to 100% of the principal amount to be redeemed together with the interest on such principal amount accrued to the date of redemption, plus a make-whole amount based on the prevailing market interest rates. The agreement requires compliance with certain covenants, including a total debt to capitalization ratio. The Company was in compliance with these requirements throughout 2017.The sale of the RGRT Notes was made by RGRT in reliance on a private placement exemption from registration under the Securities Act of 1933, as amended. The proceeds of $109.4 million, net of issuance costs, from the sale of the RGRT Notes was used by RGRT to repay amounts borrowed under the RCF and will enable future nuclear fuel financing requirements of RGRT to be met with a combination of the RGRT Notes and amounts borrowed from the RCF.

[4] Revolving Credit Facility On January 14, 2014, the Company and RGRT entered into a second amended and restated credit agreement related to the RCF with JP Morgan Chase Bank, N.A., as administrative agent and issuing bank, and Union Bank, N.A., as syndication agent, and various lending banks party thereto. As of December 31, 2016, the Company had available $300 million and the ability to increase the RCF by up to $100 million with a term ending January 2019. On January 9, 2017, the Company exercised its option to extend the maturity of the RCF by one year to January 14, 2020 and to increase the size of the facility by $50 million to $350 million. The Company still has the option to extend the facility by one additional year to January 2021 and to increase the RCF by up to $50 million (up to a total of $400 million) upon the satisfaction of certain conditions, more fully set forth in the agreement, including obtaining commitments from lenders or third party financial institutions.The RCF provides that amounts borrowed by the Company may be used for, among other things, working capital and general corporate purposes. Any amounts borrowed by RGRT may be used, among other things, to finance the acquisition and processing of nuclear fuel. Amounts borrowed by RGRT are guaranteed by the Company and the balance borrowed under the RCF is recorded as short-term borrowings on the balance sheet. The RCF is unsecured. The RCF requires compliance with certain covenants, including a total debt to capitalization ratio. The Company was in compliance with these requirements throughout 2017. In August 2015 and 2017, $15.0 million aggregate principal amount of Series A 3.67% Senior Notes and $50.0 million aggregate principal amount of Series B 4.47% Senior Notes of RGRT, respectively, matured and were paid with borrowings from the RCF. As of December 31, 2017, the total amount borrowed by RGRT was $88.5 million for nuclear fuel under the RCF. As of December 31, 2017, $85.0 million of borrowings were outstanding under this facility for working capital and general corporate purposes. The weighted average interest rate on the RCF was 2.7% as of December 31, 2017.