Borrowings and Other Financing Instruments, Commercial Paper (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Commercial Paper [Abstract]
Borrowing limit	$ 2,450,000,000	$ 2,450,000,000
Amount outstanding at period end	425,000,000	602,000,000
Average amount outstanding	813,000,000	403,000,000
Maximum amount outstanding	$ 1,160,000,000	$ 634,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.35%	0.35%
Weighted average interest rate at end of period (in hundredths)	0.33%	0.36%