Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Short-term debt	$ 136	$ 136	$ 0
Letter of Credit
Short-term Debt [Line Items]
Line of Credit Facility, Expiration Period		1 year
Commercial Paper [Member]
Short-term Debt [Line Items]
Borrowing limit	700	$ 700	700	$ 700
Short-term debt	136	136	0	307
Average amount outstanding	$ 8	$ 30	$ 154	$ 55
Weighted average interest rate at period end (percentage)	0.20%	0.20%		2.95%
Line of Credit Facility, Interest Rate During Period	0.19%	1.59%	2.67%	2.28%
Maximum amount outstanding	$ 136	$ 230	$ 432	$ 309
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250	250	250	250
Short-term debt	57	57	39	0
Average amount outstanding	$ 104	$ 59	$ 7	$ 25
Weighted average interest rate at period end (percentage)	0.07%	0.07%	1.63%
Line of Credit Facility, Interest Rate During Period	0.08%	0.60%	2.29%	1.93%
Maximum amount outstanding	$ 218	$ 250	$ 50	$ 156