Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) (First Mortgage Bonds, Series Due Aug. 15, 2041, USD $)	1 Months Ended
	Aug. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
First Mortgage Bonds | Series Due Aug. 15, 2041
Debt Instrument [Line Items]
Face amount	$ 100,000,000
Interest rate, stated percentage (in hundredths)	4.50%
Maturity date	Aug. 15, 2041
Principal outstanding		$ 400,000,000	$ 300,000,000