STATEMENTS OF CAPITALIZATION (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Long-Term Debt
Unamortized premium (discount)	$ 3,684	$ (6,686)
Total long-term debt, including current maturities	1,103,684	993,314
Less: current maturities	0	0
Total long-term debt	1,103,684	993,314
Common Stockholder's Equity
Common stock - 200 shares authorized of $1.00 par value, 100 shares outstanding at Dec. 31, 2012 and 2011, respectively	0	0
Additional paid in capital	843,186	783,162
Retained earnings	335,101	295,201
Accumulated other comprehensive loss	(1,332)	(1,504)
Total common stockholder's equity	1,176,955	1,076,859
First Mortgage Bonds [Member] | Series Due Aug. 15, 2041 [Member]
Long-Term Debt
Long-term debt, gross	300,000	200,000
Unsecured Senior Notes [Member] | Senior E Due Oct. 1, 2016 [Member]
Long-Term Debt
Long-term debt, gross	200,000	200,000
Unsecured Senior Notes [Member] | Senior G Due Dec. 1, 2018 [Member]
Long-Term Debt
Long-term debt, gross	250,000	250,000
Unsecured Senior Notes [Member] | Senior C and D Due Oct. 1, 2033 [Member]
Long-Term Debt
Long-term debt, gross	100,000	100,000
Unsecured Senior Notes [Member] | Senior F Due Oct. 1, 2036 [Member]
Long-Term Debt
Long-term debt, gross	$ 250,000	$ 250,000