STATEMENTS OF CAPITALIZATION (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Long-Term Debt
Unamortized (discount) premium	$ (135)	$ 3,684
Total long-term debt, including current maturities	1,199,865	1,103,684
Less: current maturities	0	0
Long-term debt	1,199,865	1,103,684
Common Stockholder's Equity
Common stock — 200 shares authorized of $1.00 par value; 100 shares outstanding at Dec. 31, 2013 and 2012, respectively	0	0
Additional paid in capital	1,005,463	843,186
Retained earnings	359,389	335,101
Accumulated other comprehensive loss	(1,161)	(1,332)
Total common stockholder’s equity	1,363,691	1,176,955
First Mortgage Bonds | Series Due Aug. 15, 2041
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instrument, interest rate stated percentage (in hundredths)	4.50%
Long-Term Debt
Long-term debt, gross	400,000	300,000
Debt Instrument, Maturity Date	Aug. 15, 2041
Unsecured Senior Notes | Senior E Due Oct. 1, 2016
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instrument, interest rate stated percentage (in hundredths)	5.60%
Long-Term Debt
Long-term debt, gross	200,000	200,000
Debt Instrument, Maturity Date	Oct. 01, 2016
Unsecured Senior Notes | Senior G Due Dec. 1, 2018
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instrument, interest rate stated percentage (in hundredths)	8.75%
Long-Term Debt
Long-term debt, gross	250,000	250,000
Debt Instrument, Maturity Date	Dec. 01, 2018
Unsecured Senior Notes | Senior C and D Due Oct. 1, 2033
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instrument, interest rate stated percentage (in hundredths)	6.00%
Long-Term Debt
Long-term debt, gross	100,000	100,000
Debt Instrument, Maturity Date	Oct. 01, 2033
Unsecured Senior Notes | Senior F Due Oct. 1, 2036
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instrument, interest rate stated percentage (in hundredths)	6.00%
Long-Term Debt
Long-term debt, gross	$ 250,000	$ 250,000
Debt Instrument, Maturity Date	Oct. 01, 2036