Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding at period end	$ 256,800,000	$ 0
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	6,000,000	100,000
Maximum amount outstanding	70,000,000	12,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.24%	0.36%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	700,000,000
Amount outstanding at period end	257,000,000	0
Average amount outstanding	127,000,000	38,000,000
Maximum amount outstanding	$ 261,000,000	$ 332,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.24%	0.34%
Short-term Debt, Weighted Average Interest Rate	0.29%