Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Commercial Paper (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]
Amount outstanding at period end	$ 19	$ 19	$ 65
Commercial Paper [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	150	150	150	$ 150
Amount outstanding at period end	19	19	65	51
Average amount outstanding	4	30	51	28
Maximum amount outstanding	$ 24	$ 95	$ 93	$ 103
Line of Credit Facility, Interest Rate During Period	0.17%	1.59%	2.38%	2.31%
Weighted average interest rate at period end (percentage)	0.17%	0.17%	1.97%	2.89%