Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 1,025	$ 814
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 31	$ 30
Letter of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)	1 year