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	$ 84,000,000	$ 84,000,000	$ 9,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000	100,000,000	100,000,000
Amount outstanding at period end	38,000,000	38,000,000	0	5,000,000
Average amount outstanding	65,000,000	46,000,000	10,000,000	12,000,000
Maximum amount outstanding	100,000,000	100,000,000	63,000,000	71,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.18%	0.15%	0.33%	0.35%
Weighted average interest rate at period end (in hundredths)	0.25%	0.25%		0.35%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	300,000,000	300,000,000	300,000,000	300,000,000
Amount outstanding at period end	84,000,000	84,000,000	9,000,000	0
Average amount outstanding	60,000,000	32,000,000	18,000,000	54,000,000
Maximum amount outstanding	$ 121,000,000	$ 140,000,000	$ 106,000,000	$ 161,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.30%	0.30%	0.39%	0.37%
Weighted average interest rate at period end (in hundredths)	0.27%	0.27%	0.36%