Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium), Net	$ (30)	$ (24)
Unamortized Debt Issuance Expense	(46)	(41)
Long-term Debt, Current Maturities	0	(400)
Long-term Debt	5,724	4,985
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	0
Long-term Debt, Maturities, Repayments of Principal in Year Two	300
Long-term Debt, Maturities, Repayments of Principal in Year Three	250
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	250
Deferred Finance Costs, Noncurrent, Net	46	41
First Mortgage Bonds \| Series Due Nov. 15, 2020 [Member]
Debt Instrument [Line Items]
Face amount	$ 0	400
Interest rate, stated percentage (in hundredths)	3.20%
First Mortgage Bonds \| Series Due Sept. 15, 2022 [Member]
Debt Instrument [Line Items]
Face amount	$ 300	300
Interest rate, stated percentage (in hundredths)	2.25%
First Mortgage Bonds \| Series Due March 15, 2023 [Member]
Debt Instrument [Line Items]
Face amount	$ 250	250
Interest rate, stated percentage (in hundredths)	2.50%
First Mortgage Bonds \| Series Due May 15, 2025 [Member]
Debt Instrument [Line Items]
Face amount	$ 250	250
Interest rate, stated percentage (in hundredths)	2.90%
First Mortgage Bonds \| Series Due June 15, 2028 [Member]
Debt Instrument [Line Items]
Face amount	$ 350	350
Interest rate, stated percentage (in hundredths)	3.70%
First Mortgage Bonds \| Series Due Jan. 15, 2031 [Member]
Debt Instrument [Line Items]
Face amount	$ 375	0
Interest rate, stated percentage (in hundredths)	1.90%
First Mortgage Bonds \| Series Due Sept. 1, 2037 [Member]
Debt Instrument [Line Items]
Face amount	$ 350	350
Interest rate, stated percentage (in hundredths)	6.25%
First Mortgage Bonds \| Series Due Aug. 1, 2038 [Member]
Debt Instrument [Line Items]
Face amount	$ 300	300
Interest rate, stated percentage (in hundredths)	6.50%
First Mortgage Bonds \| Series Due Aug. 15, 2041 [Member]
Debt Instrument [Line Items]
Face amount	$ 250	250
Interest rate, stated percentage (in hundredths)	4.75%
First Mortgage Bonds \| Series Due Sept. 15, 2042 [Member]
Debt Instrument [Line Items]
Face amount	$ 500	500
Interest rate, stated percentage (in hundredths)	3.60%
First Mortgage Bonds \| Series Due March 15, 2043 [Member]
Debt Instrument [Line Items]
Face amount	$ 250	250
Interest rate, stated percentage (in hundredths)	3.95%
First Mortgage Bonds \| Series Due March 15, 2044 [Member]
Debt Instrument [Line Items]
Face amount	$ 300	300
Interest rate, stated percentage (in hundredths)	4.30%
First Mortgage Bonds \| Series Due June 15, 2046 [Member]
Debt Instrument [Line Items]
Face amount	$ 250	250
Interest rate, stated percentage (in hundredths)	3.55%
First Mortgage Bonds \| Series Due June 15, 2047 [Member]
Debt Instrument [Line Items]
Face amount	$ 400	400
Interest rate, stated percentage (in hundredths)	3.80%
First Mortgage Bonds \| Series Due June 15, 2048 [Member]
Debt Instrument [Line Items]
Face amount	$ 350	350
Interest rate, stated percentage (in hundredths)	4.10%
First Mortgage Bonds \| Series Due September 15, 2049 [Domain]
Debt Instrument [Line Items]
Face amount	$ 400	400
Interest rate, stated percentage (in hundredths)	4.05%
First Mortgage Bonds \| Series Due March 1, 2050 [Domain]
Debt Instrument [Line Items]
Face amount	$ 550	550
Interest rate, stated percentage (in hundredths)	3.20%
First Mortgage Bonds \| Series Due Jan. 15, 2051 [Member]
Debt Instrument [Line Items]
Face amount	$ 375	0
Interest rate, stated percentage (in hundredths)	2.70%
Long-term Debt
Debt Instrument [Line Items]
Long-term Debt	$ 5,724	$ 4,985

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

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