Borrowings and Other Financing Instruments Short-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,038	$ 595	$ 1,038
Commercial Paper
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	3,250	3,600	3,250	$ 3,250
Amount outstanding at period end	1,038	595	1,038	814
Average amount outstanding	663	1,115	788	644
Maximum amount outstanding	$ 945	$ 1,780	$ 1,349	$ 1,247
Weighted average interest rate, computed on a daily basis (percentage)	2.40%	2.72%	2.34%	1.35%
Weighted average interest rate at period end (percentage)	2.97%	2.34%	2.97%	1.90%