STATEMENTS OF CAPITALIZATION (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Long-Term Debt
Unamortized (discount) premium	$ (309)	$ (135)
Total long-term debt, including current maturities	1,349,691us-gaap_LongTermDebt	1,199,865us-gaap_LongTermDebt
Less: current maturities	0us-gaap_LongTermDebtCurrent	0us-gaap_LongTermDebtCurrent
Long-term debt	1,349,691us-gaap_LongTermDebtNoncurrent	1,199,865us-gaap_LongTermDebtNoncurrent
Common Stockholder's Equity
Common stock — 200 shares authorized of $1.00 par value; 100 shares outstanding at Dec. 31, 2014 and 2013, respectively	0us-gaap_CommonStockValue	0us-gaap_CommonStockValue
Additional paid in capital	1,165,463	1,005,463
Retained earnings	395,998	359,389
Accumulated other comprehensive loss	(989)	(1,161)
Total common stockholder’s equity	1,560,472us-gaap_StockholdersEquity	1,363,691us-gaap_StockholdersEquity
First Mortgage Bonds \| Series Due Aug. 15, 2041
Long-Term Debt
Principal outstanding	400,000	400,000
First Mortgage Bonds \| Senior C and D Due Oct. 1, 2033
Long-Term Debt
Principal outstanding	100,000
Unsecured Senior Notes \| Senior E Due Oct. 1, 2016
Long-Term Debt
Principal outstanding	200,000	200,000
Unsecured Senior Notes \| Senior G Due Dec. 1, 2018
Long-Term Debt
Principal outstanding	250,000	250,000
Unsecured Senior Notes \| Senior C and D Due Oct. 1, 2033
Long-Term Debt
Principal outstanding	100,000	100,000
Unsecured Senior Notes \| Senior F Due Oct. 1, 2036
Long-Term Debt
Principal outstanding	250,000	250,000
SPS [Member] \| First Mortgage Bonds \| Series Due June 15, 2024
Long-Term Debt
Principal outstanding	$ 150,000	$ 0

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