Long-Term Debt and Financing Obligations Outstanding Balance (Details) $ in Thousands												1 Months Ended
		Dec. 31, 2015 USD ($)	Aug. 15, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 01, 2014 USD ($)	Jan. 14, 2014 USD ($)	Dec. 06, 2012 USD ($)	Aug. 31, 2010 USD ($)	Aug. 18, 2010 USD ($)	Jun. 02, 2008	May. 02, 2005	Jun. 29, 2008 USD ($)	Aug. 28, 2012	Jun. 01, 2008 USD ($)	May. 01, 2005 USD ($)
Debt Instrument [Line Items]
Long-term Debt		$ 1,276,022		$ 1,163,711
Long-term Debt, Current Maturities		0		(15,000)
Line of Credit, Current		(141,738)		(14,532)
Long-term debt, net of current portion		1,134,284		1,134,179
Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit		33,700		14,500
Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit		108,000		0
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		$ 141,738
Line of Credit Weighted Average Interest Rate		1.40%
Revolving Credit Facility | Nuclear Fuel Financing
Debt Instrument [Line Items]
Long-term Line of Credit		$ 33,700
Revolving Credit Facility | Working Capital and General Purpose
Debt Instrument [Line Items]
Long-term Line of Credit		108,000
Revolving Credit Facility Expiring January 2019
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date						Jan. 14, 2019
Line of Credit renewal option, renewal times						2
Line of Credit renewal term, year						1 year
Line of Credit Facility, Current Borrowing Capacity						$ 300,000
Line of Credit Facility, Maximum Borrowing Capacity						400,000
Line of Credit Facility, Increase (Decrease), Net						$ 100,000
Bonds and Notes
Debt Instrument [Line Items]
Long-term Debt		1,134,284		1,149,179
Pollution Control Bonds
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 193,135		193,135
Number of tax exempt bond series		4
Pollution Control Bonds | 7.25% 2009 Series A Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 63,500		$ 63,500
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 | 4.50% 2012 Series A Refunding Bonds, due 2042
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 59,235		$ 59,235
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
Pollution Control Bonds | 7.25% 2009 Series B Refunding Bonds, due 2040
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 37,100		$ 37,100
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 | 1.875% 2012 Series A Refunding Bonds, due 2032
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 33,300		$ 33,300
Debt Instrument, Interest Rate, Stated Percentage		1.875%		1.875%									1.875%
Debt Instrument, Interest Rate, Effective Percentage		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
Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[2]	846,149		$ 846,044
Senior Notes | 6.00% Senior Notes, Net of Discount, due 2035
Debt Instrument [Line Items]
Long-term Debt	[2]	$ 398,069		$ 398,021
Debt Instrument, Interest Rate, Stated Percentage		6.00%		6.00%											6.00%
Debt Instrument, Interest Rate, Effective Percentage		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	[2]	$ 148,838		$ 148,818
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 | 3.30% Senior Notes, Net of Discount, due 2022
Debt Instrument [Line Items]
Long-term Debt	[2]	$ 149,766		$ 149,737
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. 06, 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]	$ 149,476		$ 149,468
Debt Instrument, Interest Rate, Stated Percentage		5.00%		5.00%	5.00%
Debt Instrument, Interest Rate, Effective Percentage		5.10%		5.10%
Debt Instrument, Maturity Date		Dec. 01, 2044		Dec. 01, 2044
Debt Instrument, Face Amount					$ 150,000
Debt Instrument, Issuance Date					Dec. 01, 2014
Debt Instrument, Unamortized Discount					$ 500
RGRT Senior Notes
Debt Instrument [Line Items]
Long-term Debt	[3]	$ 95,000		$ 110,000
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	[3]	$ 0		$ 15,000
Long-term Debt, Current Maturities			$ (15,000)
Debt Instrument, Interest Rate, Stated Percentage		3.67%		3.67%
Debt Instrument, Interest Rate, Effective Percentage		3.87%		3.87%
Debt Instrument, Maturity Date			Aug. 15, 2015	Aug. 15, 2015
RGRT Senior Notes | 4.47% Senior Notes, Series B, due 2017
Debt Instrument [Line Items]
Long-term Debt	[3]	$ 50,000		$ 50,000
Debt Instrument, Interest Rate, Stated Percentage		4.47%		4.47%
Debt Instrument, Interest Rate, Effective Percentage		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	[3]	$ 45,000		$ 45,000
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
Financing Obligations | Revolving Credit Facility
Debt Instrument [Line Items]
Long-term Line of Credit	[4]	$ 141,738		$ 14,532

[1] 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.

[2] 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 and 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.The 5.00% Senior Notes have an aggregate principal amount of $150.0 million and were issued in December 2014. The proceeds, net of a $0.5 million discount, were used to fund construction expenditures and for working capital and general corporate purposes.

[3] 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. The Company guarantees the payment of principal and interest on the Notes. In the Company’s financial statements, the assets and liabilities of the 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 2015.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 revolving credit facility 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.

[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. Under the terms of the agreement, the Company has available $300 million and the ability to increase the RCF by up to $100 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 has a term ending January 2019. 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.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 2015. 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 RFC. As of December 31, 2015, the total amount borrowed by RGRT was $33.7 million for nuclear fuel under the RCF. As of December 31, 2015, $108.0 million of borrowings were outstanding under this facility for working capital and general corporate purposes. The weighted average interest rate on the RCF was 1.4% as of December 31, 2015.