Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 0	$ 154,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	700,000,000	700,000,000	700,000,000
Amount outstanding at period end	0	0	154,000,000	0
Average amount outstanding	0	38,000,000	8,000,000	73,000,000
Maximum amount outstanding		332,000,000	165,000,000	304,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)		0.34%	0.33%	0.37%
Weighted average interest rate at period end (in hundredths)			0.35%
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000	250,000,000	250,000,000
Borrowings under utility money pool arrangement	0	0	0	0
Average amount outstanding	0	100,000	300,000	3,000,000
Maximum amount outstanding		$ 12,000,000	$ 8,000,000	$ 53,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)		0.36%	0.33%	0.35%