STATEMENTS OF CAPITALIZATION - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Capitalization, Long-term Debt [Line Items]
Unamortized discount (premium), net	$ (1,746)	$ 365
Unamortized Debt Issuance Expense	(18,313)	(14,507)
Long-term debt	1,829,941	1,635,858
Common Stockholder's Equity
Common stock — 200 shares authorized of $1.00 par value; 100 shares outstanding at Dec. 31, 2017 and 2016, respectively	0	0
Additional paid in capital	1,590,242	1,446,223
Retained earnings	541,588	486,763
Accumulated other comprehensive loss	(1,467)	(1,290)
Total common stockholder’s equity	2,130,363	1,931,696
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	300,000
First Mortgage Bonds, Series due: | Series Due August 15, 2047
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	450,000	0
Unsecured Senior Notes | Senior G Due Dec. 1, 2018
Schedule of Capitalization, Long-term Debt [Line Items]
Principal outstanding	0	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