Borrowings and Other Financing Instruments, Commercial Paper (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Short-term Debt [Line Items]
Amount outstanding at period end	$ 39,000,000	$ 39,000,000	$ 66,000,000
Commercial Paper [Member]
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000	150,000,000
Amount outstanding at period end	39,000,000	39,000,000	66,000,000
Average amount outstanding	21,000,000	61,000,000	24,000,000
Maximum amount outstanding	$ 101,000,000	$ 116,000,000	$ 70,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.39%	0.39%	0.37%
Weighted average interest rate at end of period (in hundredths)	0.40%	0.40%	0.46%