Borrowings and Other Financing Instruments, Commercial Paper (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Commercial Paper [Abstract]
Borrowing limit	$ 2,450,000,000	$ 2,450,000,000
Amount outstanding at period end	354,000,000	602,000,000
Average amount outstanding	255,000,000	403,000,000
Maximum amount outstanding	$ 487,000,000	$ 634,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.29%	0.35%
Weighted average interest rate at period end (in hundredths)	0.27%	0.36%