CONSOLIDATED STATEMENTS OF CAPITALIZATION (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Unamortized Discount (Premium), Net	$ (11,301)		$ (9,468)
Total long-term debt	3,872,643		3,630,773
Less current maturities	282,143		256,297
Long-term Debt and Capital Lease Obligations	3,590,500		3,374,476
Common Stockholder's Equity
Common stock - 100 shares authorized of $0.01 par value, 100 shares outstanding at Dec. 31, 2013 and 2012, respectively	0		0
Additional paid in capital	3,441,290		3,415,669
Retained earnings	1,384,047		1,192,937
Accumulated other comprehensive loss	(23,338)		(22,871)
Total common stockholder’s equity	4,801,999		4,585,735
First Mortgage Bonds | Series Due March 1, 2013
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	0		250,000
First Mortgage Bonds | Series Due April 1, 2014
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	275,000		275,000
First Mortgage Bonds | Series Due Sept. 1, 2017
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	129,500	[1]	129,500	[1]
First Mortgage Bonds | Series Due Aug. 1, 2018
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	300,000		300,000
First Mortgage Bonds | Series Due June 1, 2019
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	400,000		400,000
First Mortgage Bonds | Series Due Nov. 15, 2020
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	400,000		400,000
First Mortgage Bonds | Series Due Sept. 15, 2022
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	300,000		300,000
First Mortgage Bonds | Series Due March 15, 2023
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	250,000		0
First Mortgage Bonds | Series Due Sept. 1, 2037
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	350,000		350,000
First Mortgage Bonds | Series Due Aug. 1, 2038
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	300,000		300,000
First Mortgage Bonds | Series Due Aug. 15, 2041
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	250,000		250,000
First Mortgage Bonds | Series Due Sept. 15, 2042
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	500,000		500,000
First Mortgage Bonds | Series Due March 15, 2043
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	250,000		0
Capital Lease Obligations
Schedule of Capitalization, Long-term Debt [Line Items]
Capital lease obligations	$ 179,444		$ 185,741

[1] Pollution control financing.