Long-Term Debt and Financing Obligations Outstanding Balance (Details) $ in Thousands															1 Months Ended	12 Months Ended
		Jan. 10, 2017	Jan. 09, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 24, 2016 USD ($)	Dec. 31, 2015 USD ($)		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, 2016 USD ($)	Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)	Aug. 28, 2012	Jun. 01, 2008 USD ($)	May 01, 2005 USD ($)
Debt Instrument [Line Items]
Long-term Debt				$ 1,360,230		$ 1,264,398	[1]									$ 1,360,230	$ 1,264,398	[1]
Long-term Debt, Current Maturities				(83,143)		0	[1]									(83,143)	0	[1]
Line of Credit, Current				(81,574)		(141,738)	[1]									(81,574)	(141,738)	[1]
Long-term debt, net of current portion				1,195,513		1,122,660	[1]									1,195,513	1,122,660	[1]
Proceeds from issuance of senior notes																157,052	0		$ 149,468
Interest accrued				13,287		12,649										13,287	12,649
Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit				37,600		33,700										37,600	33,700
Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit				44,000		108,000										44,000	108,000
Short Term Borrowing
Debt Instrument [Line Items]
Repayments of Short-term Debt															$ 44,000
Revolving Credit Facility
Debt Instrument [Line Items]
Long-term Line of Credit				$ 81,574												$ 81,574
Line of Credit Weighted Average Interest Rate				2.00%												2.00%
Revolving Credit Facility | Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit				$ 37,600												$ 37,600
Revolving Credit Facility | Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit				$ 44,000												44,000
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
Bonds and Notes
Debt Instrument [Line Items]
Long-term Debt				1,278,656		1,122,660	[1]									1,278,656	1,122,660	[1]
Pollution Control Bonds
Debt Instrument [Line Items]
Long-term Debt	[2]			$ 190,775		190,499	[1]									$ 190,775	190,499	[1]
Number of tax exempt bond series				4												4
Debt Instrument, Face Amount				$ 193,100												$ 193,100
Pollution Control Bonds | 7.25% 2009 Series A Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[2]			$ 62,619		$ 62,582	[1]									$ 62,619	$ 62,582	[1]
Debt Instrument, Interest Rate, Stated Percentage				7.25%		7.25%										7.25%	7.25%
Debt Instrument, Interest Rate, Effective Percentage				7.46%		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	[2]			$ 58,471		$ 58,441	[1]									$ 58,471	$ 58,441	[1]
Debt Instrument, Interest Rate, Stated Percentage				4.50%		4.50%										4.50%	4.50%
Debt Instrument, Interest Rate, Effective Percentage				4.63%		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	[2]			$ 36,492		$ 36,465	[1]									$ 36,492	$ 36,465	[1]
Debt Instrument, Interest Rate, Stated Percentage				7.25%		7.25%										7.25%	7.25%
Debt Instrument, Interest Rate, Effective Percentage				7.49%		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	[2]			$ 33,193		$ 33,011	[1]									$ 33,193	$ 33,011	[1]
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%	2.35%
Debt Instrument, Maturity Date				Jun. 01, 2032		Jun. 01, 2032
Debt Instrument Remarketed Date, Year				Sep. 30, 2017
Debt Instrument, Face Amount				$ 33,300												$ 33,300
Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[3]			993,086		$ 837,475	[1]									993,086	$ 837,475	[1]
Senior Notes | 6.00% Senior Notes, Net of Discount, due 2035
Debt Instrument [Line Items]
Long-term Debt	[3]			$ 393,861		$ 393,693	[1]									$ 393,861	$ 393,693	[1]
Debt Instrument, Interest Rate, Stated Percentage				6.00%		6.00%										6.00%	6.00%					6.00%
Debt Instrument, Interest Rate, Effective Percentage				7.12%		7.12%										7.12%	7.12%
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	[3]			$ 147,331		$ 147,282	[1]									$ 147,331	$ 147,282	[1]
Debt Instrument, Interest Rate, Stated Percentage				7.50%		7.50%										7.50%	7.50%				7.50%
Debt Instrument, Interest Rate, Effective Percentage				7.67%		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	[3]			$ 148,939		$ 148,783	[1]									$ 148,939	$ 148,783	[1]
Debt Instrument, Interest Rate, Stated Percentage				3.30%		3.30%				3.30%						3.30%	3.30%
Debt Instrument, Interest Rate, Effective Percentage				3.43%		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	[3]			$ 302,955		$ 147,717	[1]									$ 302,955	$ 147,717	[1]
Debt Instrument, Interest Rate, Stated Percentage				5.00%	5.00%	5.00%										5.00%	5.00%
Debt Instrument, Interest Rate, Effective Percentage				4.93%		4.93%										4.93%	4.93%
Debt Instrument, Maturity Date				Dec. 01, 2044	Dec. 01, 2044	Dec. 01, 2044
Debt Instrument, Face Amount				$ 300,000	$ 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	[4]			94,795		$ 94,686	[1]									94,795	$ 94,686	[1]
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	[4]			49,950		$ 49,883	[1]									49,950	$ 49,883	[1]
Long-term Debt, Current Maturities				$ (50,000)												$ (50,000)
Debt Instrument, Interest Rate, Stated Percentage				4.47%		4.47%										4.47%	4.47%
Debt Instrument, Interest Rate, Effective Percentage				4.62%		4.62%										4.62%	4.62%
Debt Instrument, Maturity Date				Aug. 15, 2017		Aug. 15, 2017
RGRT Senior Notes | 5.04% Senior Notes, Series C, due 2020
Debt Instrument [Line Items]
Long-term Debt	[4]			$ 44,845		$ 44,803	[1]									$ 44,845	$ 44,803	[1]
Debt Instrument, Interest Rate, Stated Percentage				5.04%		5.04%										5.04%	5.04%
Debt Instrument, Interest Rate, Effective Percentage				5.16%		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	[5]			$ 81,574		$ 141,738	[1]									$ 81,574	$ 141,738	[1]
Subsequent Event
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date		Jan. 14, 2021
Line of Credit renewal term, year		1 year
Subsequent Event | 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

[1] The Company implemented ASU 2015-03, Interest - Imputation of Interest, which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability. ASU 2015-03 is effective for financial statements issued for fiscal years beginning after December 15, 2015, and interim periods within those fiscal years. The impact of ASU 2015-03 on the Company's Balance Sheet was to reclassify $11.6 million of other deferred charges to long-term debt, net of current portion at December 31, 2015.

[2] Pollution Control Bonds ("PCBs")The Company has four series of tax exempt unsecured PCBs in aggregate principal amount of $193.1 million. The 1.875% 2012 Series A (El Paso Electric Company Four Corners Project) Pollution Control Refunding Revenue Bonds with an aggregate principal amount of $33.3 million are subject to mandatory tender for purchase in September 2017 at which time the Company will either repay or remarket these bonds.

[3] Senior NotesThe 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.

[4] RGRT Senior NotesIn 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 (the "Notes"). In August 2015, $15.0 million of these Notes matured and were paid with borrowings from the RCF. In August 2017, $50.0 million of these Senior Notes will mature. The Company will either repay or refinance this $50.0 million of Notes upon maturity. The Company guarantees the payment of principal and interest on the 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 Notes on February 15, and August 15 of each year until maturity. RGRT 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 agreement requires compliance with certain covenants, including a total debt to capitalization ratio. The Company was in compliance with these requirements throughout 2016.The sale of the 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 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 Notes and amounts borrowed from the RCF.

[5] 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. 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 2016. In August 2015, $15.0 million aggregate principal amount of Series A 3.67% Senior Notes of RGRT matured and were paid utilizing borrowings under the RCF. As of December 31, 2016, the total amount borrowed by RGRT was $37.6 million for nuclear fuel under the RCF. As of December 31, 2016, $44.0 million of borrowings were outstanding under this facility for working capital and general corporate purposes. The weighted average interest rate on the RCF was 2.0% as of December 31, 2016.