Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Money Pool
Short-term Debt [Line Items]
Borrowing limit	$ 250,000,000	$ 250,000,000
Amount outstanding at period end	0	0
Average amount outstanding	0	100,000
Maximum amount outstanding	2,000,000	12,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.21%	0.36%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	700,000,000
Amount outstanding at period end	0	0
Average amount outstanding	20,000,000	38,000,000
Maximum amount outstanding	$ 114,000,000	$ 332,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.21%	0.34%