Borrowings and Other Financing Instruments, Commercial Paper (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Commercial Paper [Abstract]
Borrowing limit	$ 2,450,000,000	$ 2,450,000,000
Amount outstanding at period end	777,800,000	759,000,000
Average amount outstanding	910,000,000	481,000,000
Maximum amount outstanding	$ 1,186,000,000	$ 1,160,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.32%	0.31%
Weighted average interest rate at period end (in hundredths)	0.35%	0.25%