CONSOLIDATED STATEMENTS OF CAPITALIZATION (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Common Stockholder's Equity
Common stock, shares authorized (in shares)	100	100
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares outstanding (in shares)	100	100
First Mortgage Bonds | Series Due Sept. 1, 2017
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	4.375%	4.375%
Debt instrument, maturity date	Sep. 01, 2017	Sep. 01, 2017
First Mortgage Bonds | Series Due Aug. 1, 2018
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	5.80%	5.80%
Debt instrument, maturity date	Aug. 01, 2018	Aug. 01, 2018
First Mortgage Bonds | Series Due June 1, 2019
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	5.125%	5.125%
Debt instrument, maturity date	Jun. 01, 2019	Jun. 01, 2019
First Mortgage Bonds | Series Due Nov. 15, 2020
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	3.20%	3.20%
Debt instrument, maturity date	Nov. 15, 2020	Nov. 15, 2020
First Mortgage Bonds | Series Due Sept. 15, 2022
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	2.25%	2.25%
Debt instrument, maturity date	Sep. 15, 2022	Sep. 15, 2022
First Mortgage Bonds | Series Due March 15, 2023
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	2.50%	2.50%
Debt instrument, maturity date	Mar. 15, 2023	Mar. 15, 2023
First Mortgage Bonds | Series Due May 15, 2025 [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	2.90%
Debt instrument, maturity date	May 15, 2025
First Mortgage Bonds | Series Due Sept. 1, 2037
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	6.25%	6.25%
Debt instrument, maturity date	Sep. 01, 2037	Sep. 01, 2037
First Mortgage Bonds | Series Due Aug. 1, 2038
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	6.50%	6.50%
Debt instrument, maturity date	Aug. 01, 2038	Aug. 01, 2038
First Mortgage Bonds | Series Due Aug. 15, 2041
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	4.75%	4.75%
Debt instrument, maturity date	Aug. 15, 2041	Aug. 15, 2041
First Mortgage Bonds | Series Due Sept. 15, 2042
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	3.60%	3.60%
Debt instrument, maturity date	Sep. 15, 2042	Sep. 15, 2042
First Mortgage Bonds | Series Due March 15, 2043
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	3.95%	3.95%
Debt instrument, maturity date	Mar. 15, 2043	Mar. 15, 2043
First Mortgage Bonds | Series Due March 15, 2044
Schedule of Capitalization, Long-term Debt [Line Items]
Interest rate, stated percentage (in hundredths)	4.30%	4.30%
Debt instrument, maturity date	Mar. 15, 2044	Mar. 15, 2044
Capital Lease Obligations
Schedule of Capitalization, Long-term Debt [Line Items]
Debt instrument, interest rate, stated percentage rate range, minimum (in hundredths)	11.20%
Debt instrument, interest rate, stated percentage rate range, maximum (in hundredths)	14.30%
Debt instrument, maturity date range, end	Dec. 31, 2060