STATEMENTS OF CAPITALIZATION - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Capitalization, Long-term Debt [Line Items]
Unamortized discount (premium), net	$ 365	$ 605
Unamortized Debt Issuance Expense	(14,507)	(12,083)
Total long-term debt, including current maturities	1,635,858	1,538,522
Less: current maturities	0	200,000
Long-term debt	1,635,858	1,338,522
Common Stockholder's Equity
Common stock — 200 shares authorized of $1.00 par value; 100 shares outstanding at Dec. 31, 2016 and 2015, respectively	0	0
Additional paid in capital	1,446,223	1,371,223
Retained earnings	486,763	438,007
Accumulated other comprehensive loss	(1,290)	(1,281)
Total common stockholder’s equity	1,931,696	1,807,949
First Mortgage Bonds, Series due: | Series Due June 15, 2024
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	350,000	350,000
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	400,000	400,000
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2046
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	300,000	0
Unsecured Senior Notes | Senior E Due Oct. 1, 2016
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	0	200,000
Unsecured Senior Notes | Senior G Due Dec. 1, 2018
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	250,000	250,000
Unsecured Senior Notes | Senior C and D Due Oct. 1, 2033
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	100,000	100,000
Unsecured Senior Notes | Senior F Due Oct. 1, 2036
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	$ 250,000	$ 250,000