Borrowings and Other Financing Instruments, Commercial Paper (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding at period end	$ 697,000,000	$ 759,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	2,450,000,000	2,450,000,000
Amount outstanding at period end	697,000,000	759,000,000
Average amount outstanding	730,000,000	481,000,000
Maximum amount outstanding	$ 894,000,000	$ 1,160,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.33%	0.31%
Weighted average interest rate at period end (percentage)	0.33%	0.25%