Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,038	$ 1,038	$ 814
Commercial Paper
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	3,250	3,250	3,250	$ 2,750
Amount outstanding at period end	1,038	1,038	814	392
Average amount outstanding	500	788	644	485
Maximum amount outstanding	$ 1,038	$ 1,349	$ 1,247	$ 1,183
Weighted average interest rate, computed on a daily basis (percentage)	2.76%	2.34%	1.35%	0.74%
Weighted average interest rate at period end (percentage)	2.97%	2.97%	1.90%	0.95%
364-Day Term Loan | Xcel Energy Inc.
Short-term Debt [Line Items]
Line of Credit Facility, Expiration Period		364 days
Amount outstanding at period end	$ 250	$ 250
364-Day Term Loan | Xcel Energy Inc.
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 500	$ 500