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	$ 8,000,000	$ 68,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	8,000,000	68,000,000
Average amount outstanding	7,000,000	20,000,000
Maximum amount outstanding	$ 22,000,000	$ 71,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.24%	0.31%
Weighted average interest rate at period end (percentage)	0.25%	0.27%