Long-Term Debt and Financing Obligations Outstanding Balance (Details) $ in Thousands																	1 Months Ended	12 Months Ended
		Feb. 01, 2019 USD ($)	Sep. 13, 2018 USD ($)	Sep. 12, 2018	Jun. 28, 2018 USD ($)	Aug. 15, 2017 USD ($)	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 ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Aug. 15, 2020 USD ($)	Jun. 01, 2008 USD ($)	May 01, 2005 USD ($)
Debt Instrument [Line Items]
Long-term Debt																		$ 1,434,426	$ 1,369,521
Long-term Debt, Current Maturities	[1]																	99,239	0
Line of Credit, Current																		(49,207)	(173,533)
Long-term debt, net of current portion																		1,285,980	1,195,988
Long-term Debt, subject for redemption																		100,600
Interest accrued																		16,478	12,470
Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit																		26,200	88,500
Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit																		$ 23,000	85,000
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																		3.80%
Revolving Credit Facility | Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit																		$ 26,200
Revolving Credit Facility | Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit																		23,000
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
Line of Credit Facility, Remaining Borrowing Capacity								$ 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, Increase (Decrease), Net							$ 50,000
Revolving Credit Faclity Expiring September 2023
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date			Sep. 13, 2023	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 Renewal Option, Renewal Times			2
Subfacility for the Issuance of Letters of Credit
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity			$ 50,000
Bonds and Notes
Debt Instrument [Line Items]
Long-term Debt																		1,385,219	1,195,988
Pollution Control Bonds
Debt Instrument [Line Items]
Long-term Debt	[1]																	$ 157,769	157,676
Number of tax exempt bond series																		3
Debt Instrument, Face Amount																		$ 159,800
Pollution Control Bonds | 7.25% 2009 Series A Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[1]																	$ 62,695	$ 62,657
Debt Instrument, Interest Rate, Stated Percentage																		7.25%	7.25%
Debt Instrument, Interest Rate, Effective Percentage																		7.46%	7.46%
Debt Instrument, Maturity Date																		Feb. 01, 2040	Feb. 01, 2040
Pollution Control Bonds | 7.25% 2009 Series B Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[1]																	$ 36,544	$ 36,518
Debt Instrument, Interest Rate, Stated Percentage																		7.25%	7.25%
Debt Instrument, Interest Rate, Effective Percentage																		7.49%	7.49%
Debt Instrument, Maturity Date																		Apr. 01, 2040	Apr. 01, 2040
Pollution Control Bonds | 4.50% 2012 Series A Refunding Bonds, due 2042
Debt Instrument [Line Items]
Long-term Debt	[1]																	$ 58,530	$ 58,501
Debt Instrument, Interest Rate, Stated Percentage																		4.50%	4.50%
Debt Instrument, Interest Rate, Effective Percentage																		4.63%	4.63%
Debt Instrument, Maturity Date																		Aug. 01, 2042	Aug. 01, 2042
Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[2]																	$ 1,117,943	$ 993,426
Senior Notes | 3.30% Senior Notes, Net of Discount, due 2022
Debt Instrument [Line Items]
Long-term Debt	[2]																	$ 149,269	$ 149,101
Debt Instrument, Interest Rate, Stated Percentage												3.30%						3.30%	3.30%
Debt Instrument, Interest Rate, Effective Percentage																		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 | 6.00% Senior Notes, Net of Discount, due 2035
Debt Instrument [Line Items]
Long-term Debt	[2]																	$ 394,231	$ 394,040
Debt Instrument, Interest Rate, Stated Percentage																		6.00%	6.00%			6.00%
Debt Instrument, Interest Rate, Effective Percentage																		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]																	$ 147,441	$ 147,384
Debt Instrument, Interest Rate, Stated Percentage																		7.50%	7.50%		7.50%
Debt Instrument, Interest Rate, Effective Percentage																		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 | 5.00% Senior Notes, Net of Discount, due 2044
Debt Instrument [Line Items]
Long-term Debt	[2]																	$ 302,845	$ 302,901
Debt Instrument, Interest Rate, Stated Percentage									5.00%									5.00%	5.00%
Debt Instrument, Interest Rate, Effective Percentage																		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
Senior Notes | 4.22% Senior Notes, due 2028
Debt Instrument [Line Items]
Long-term Debt	[2]																	$ 124,157	$ 0
Debt Instrument, Interest Rate, Stated Percentage					4.22%													4.22%	4.22%
Debt Instrument, Interest Rate, Effective Percentage																		4.30%	4.30%
Debt Instrument, Maturity Date					Aug. 15, 2028													Jun. 28, 2028	Jun. 28, 2028
Debt Instrument, Face Amount					$ 125,000
Redemption Price of Notes Equal to Principal Amount, Percentage					100.00%
Debt Instrument, Issuance Date					Jun. 28, 2018
Senior Notes - Public
Debt Instrument [Line Items]
Long-term Debt	[2]																	$ 993,786	$ 993,426
RGRT Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[3]																	109,507	44,886
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 | 5.04% Senior Notes, Series C, due 2020
Debt Instrument [Line Items]
Long-term Debt	[3]																	$ 44,928	$ 44,886
Debt Instrument, Interest Rate, Stated Percentage																		5.04%	5.04%
Debt Instrument, Interest Rate, Effective Percentage																		5.16%	5.16%
Debt Instrument, Maturity Date																		Aug. 15, 2020	Aug. 15, 2020
RGRT Senior Notes | 4.07% RGRT Senior Notes, due 2025
Debt Instrument [Line Items]
Long-term Debt	[3]																	$ 64,579	$ 0
Debt Instrument, Interest Rate, Stated Percentage					4.07%													4.07%	4.07%
Debt Instrument, Interest Rate, Effective Percentage																		4.18%	4.18%
Debt Instrument, Maturity Date					Aug. 15, 2025													Aug. 15, 2025	Aug. 15, 2025
Debt Instrument, Face Amount					$ 65,000
Debt Instrument, Issuance Date					Jun. 28, 2018
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, Current Maturities						$ 50,000
Debt Instrument, Interest Rate, Stated Percentage						4.47%
Debt Instrument, Maturity Date						Aug. 15, 2017
Financing Obligations | Revolving Credit Facility
Debt Instrument [Line Items]
Long-term Line of Credit	[4]																	$ 49,207	$ 173,533
Scenario, Forecast | RGRT Senior Notes | 5.04% Senior Notes, Series C, due 2020
Debt Instrument [Line Items]
Long-term Debt, Current Maturities																				$ 45,000
Subsequent Event
Debt Instrument [Line Items]
Debt Instrument, Repurchase Date		Feb. 01, 2019
Subsequent Event | Pollution Control Bonds | 7.25% 2009 Series A Refunding Bonds, due 2040
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount		$ 63,500

[1] Pollution Control BondsThe Company has three series of tax exempt unsecured PCBs in aggregate principal amount of $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, at which time the Company expects to repay, remarket or replace these bonds. The principal and related unamortized issuance cost on these PCBs were reclassified to current maturities of long-term debt as of December 31, 2018. On February 1, 2019, the Company purchased in lieu of redemption all of the 7.25% 2009 Series A with a principal amount of $63.5 million utilizing funds borrowed under the RCF. The Company is currently holding the bonds and may remarket them or replace them with debt instruments of equivalent value at a future date depending on financing needs and market conditions.

[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 P of Notes to Financial Statements. 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 outstanding 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 repay outstanding short-term borrowings, 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. On June 28, 2018, the Company entered into a note purchase agreement with several institutional purchasers under which the Company issued and sold $125 million aggregate principal amount of 4.22% Senior Notes due August 15, 2028. The net proceeds from the issuance of these senior notes were used to repay outstanding short-term borrowings under the RCF for working capital and general corporate purposes. The Company will pay interest on the notes semi-annually on February 15 and August 15 of each year until maturity, beginning on February 15, 2019. The Company may redeem the 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 note purchase agreement requires compliance with certain covenants, including a total debt to capitalization ratio. The Company was in compliance with these requirements throughout 2018. The issuance and sale of these senior notes was made in reliance on a private placement exemption from the registration provisions of the Securities Act of 1933, as amended ("Securities Act").

[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 issued and 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 the 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. In August 2020, the remaining $45.0 million of these RGRT Notes mature.The sale of the RGRT Notes was made by RGRT in reliance on a private placement exemption from registration under the Securities Act. 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 enabled future nuclear fuel financing requirements of RGRT to be met with a combination of the RGRT Notes and amounts borrowed from the RCF.On June 28, 2018, the RGRT and the Company entered into a note purchase agreement with several institutional purchasers under which the RGRT issued and sold $65 million aggregate principal amount of 4.07% Senior Guaranteed Notes due August 15, 2025 ("RGRT Senior Notes"). The net proceeds from the RGRT Senior Notes were used to repay outstanding short-term borrowings under the RCF to finance nuclear fuel purchases. The Company guaranteed the payment of principal and interest on the RGRT Senior Notes. RGRT’s assets, liabilities and operations are consolidated in the Company’s financial statements and the RGRT Senior Notes are included as long-term debt on the balance sheet. The issuance and sale of the RGRT Senior Notes was made in reliance on a private placement exemption from the registration provisions of the Securities Act.RGRT pays interest on the senior notes above on February 15 and August 15 of each year until maturity, beginning on February 15, 2019. RGRT may redeem the senior 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 note purchase agreement requires compliance with certain covenants, including a total debt to capitalization ratio. The Company and RGRT were in compliance with these requirements throughout 2018.

[4] Revolving Credit FacilityOn 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. On September 13, 2018, the Company and The Bank of New York Mellon Trust Company, N.A., as trustee of the RGRT, entered into a third amended and restated credit agreement ("RCF Agreement") with MUFG Union Bank, N.A., as administrative agent and as syndication agent, various issuing banks and lending banks party thereto. Under the terms of the RCF Agreement, the Company has available a $350 million RCF with a $50 million subfacility for the issuance of letters of credit, and the Company extended the term of the Company's existing $350 million revolving credit agreement from January 14, 2020 to September 13, 2023 ("Maturity Date"). The Company may increase the RCF by up to $50 million (to a total of $400 million) during the term of the RCF Agreement, upon the satisfaction of certain conditions more fully set forth in the RCF Agreement, including obtaining commitments from lenders or third party financial institutions. In addition, the Company may extend the Maturity Date up to two times, in each case for an additional one-year period, upon the satisfaction of certain conditions more fully set forth in the RCF Agreement, including requisite lender approval.The RCF Agreement provides that amounts borrowed by the Company may be used for, among other things, working capitaland general corporate purposes. Any amounts borrowed by the RGRT may be used, among other things, to finance the acquisition and cost to process nuclear fuel. Amounts borrowed by the RGRT are guaranteed by the Company and the balance borrowed under the RCF Agreement is recorded as short-term borrowings on the balance sheet. The RCF Agreement is unsecured. The RCF Agreement requires compliance with certain covenants, including a total debt to capitalization ratio. The Company is in compliance with these requirements throughout 2018. In August 2017, $50.0 million aggregate principal amount of Series B 4.47% Senior Notes of the RGRT matured and was paid with borrowings from the RCF. On February 1, 2019, the Company purchased in lieu of redemption all of the 7.25% 2009 Series A PCBs with a principal amount of $63.5 million utilizing funds borrowed under the RCF. The Company is currently holding the bonds and may remarket them or replace them with debt instruments of equivalent value at a future date depending on the Company's financing needs and market conditions. As of December 31, 2018, the total amount borrowed by the RGRT was $26.2 million for nuclear fuel under the RCF. As of December 31, 2018, $23.0 million of borrowings were outstanding under this facility for working capital and general corporate purposes. The weighted average interest rate on the RCF was 3.8% as of December 31, 2018.