CONSOLIDATED STATEMENTS OF CAPITALIZATION - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Schedule of Capitalization, Long-term Debt [Line Items]
Unamortized Debt Issuance Expense		$ (26,799)	$ (26,595)
Debt Instrument, Unamortized Discount (Premium), Net		(12,922)	(11,340)
Total long-term debt		4,216,206	4,105,596
Less current maturities		5,270	8,103
Long-term Debt and Capital Lease Obligations		4,210,936	4,097,493
Common Stockholder's Equity
Common Stock — 100 shares authorized of $0.01 par value; 100 shares outstanding at Dec. 31, 2016 and 2015, respectively.		0	0
Additional paid in capital		3,633,216	3,620,824
Retained earnings		1,659,239	1,523,164
Accumulated other comprehensive loss		(23,000)	(23,836)
Total common stockholder’s equity		5,269,455	5,120,152
First Mortgage Bonds | Series Due Sept. 1, 2017
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross	[1]	0	129,500
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	250,000
First Mortgage Bonds | Series Due May 15, 2025
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross		250,000	250,000
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	250,000
First Mortgage Bonds | Series Due March 15, 2044
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term debt, gross		300,000	300,000
First Mortgage Bonds | Series Due June 15, 2046 [Member]
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		$ 155,927	$ 164,031

[1] Pollution control financing.