Long-Term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium), Net	$ (10)	$ (7)
Unamortized Debt Issuance Expense	(26)	(23)
Deferred Finance Costs, Noncurrent, Net	26	23
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	0
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	0
Long-term Debt, Maturities, Repayments of Principal in Year Four	350
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Excluding Current Maturities	$ 2,764	2,420
First Mortgage Bonds, Series due: \| Series Due June 15, 2024 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.30%
Face Amount	$ 150	150
First Mortgage Bonds, Series due: \| Series Due June 15, 2024 2 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.30%
Face Amount	$ 200	200
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2041 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	4.50%
Face Amount	$ 200	200
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2041 2 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	4.50%
Face Amount	$ 100	100
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2041 3 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	4.50%
Face Amount	$ 100	100
First Mortgage Bonds, Series due: \| Series Due August 15, 2046 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.40%
Face Amount	$ 300	300
First Mortgage Bonds, Series due: \| Series Due August 15, 2047 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.70%
Face Amount	$ 450	450
First Mortgage Bonds, Series due: \| Series Due Nov. 15, 2048 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	4.40%
Face Amount	$ 300	300
First Mortgage Bonds, Series due: \| Series Due June 15, 2049 [Domain]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.75%
Face Amount	$ 300	300
First Mortgage Bonds, Series due: \| Series due May 1, 2050 [Member] \| SPS [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.15%
Face Amount	$ 350	0
Unsecured Debt [Member] \| Senior C and D Due Oct. 1, 2033 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	6.00%
Face Amount	$ 100	100
Unsecured Debt [Member] \| Senior F Due Oct. 1, 2036 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	6.00%
Face Amount	$ 250	250
Long-term Debt [Member]
Debt Instrument [Line Items]
Long-term Debt and Lease Obligation	$ 2,764	$ 2,420

Face (par) amount of debt instrument at time of issuance.

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