Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) (USD $)	1 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Aug. 31, 2011
Borrowings and Other Financing Instruments [Abstract]
Long-term debt maturities due in 2016		$ 200,000,000
Deferred financing costs		9,700,000	8,900,000
Dividend Restrictions [Abstract]
Equity-to-total capitalization ratio (excluding short-term debt), low end of range (in hundredths)		45.00%
Equity-to-total capitalization ratio (excluding short-term debt), high end of range (in hundredths)		55.00%
Equity-to-total capitalization ratio (excluding short-term debt) (in hundredths)		51.60%
First Mortgage Bonds [Member] | Series Due Aug. 15, 2041 [Member]
Borrowings and Other Financing Instruments [Abstract]
Face amount	100,000,000			200,000,000
Interest rate, stated percentage (in hundredths)	4.50%	4.50%
Maturity date	Aug. 15, 2041	Aug. 15, 2041
Principal outstanding		$ 300,000,000	$ 200,000,000