Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Oct. 31, 2012 First Mortgage Bonds [Member] Series Due Oct. 1, 2042 [Member]	Dec. 31, 2012 First Mortgage Bonds [Member] Series Due Oct. 1, 2042 [Member]
Long-Term Borrowings and Other Financing Instruments [Abstract]
Face amount			$ 100
Interest rate, stated percentage (in hundredths)			3.70%	3.70%
Maturity date			Oct. 01, 2042	Oct. 01, 2042
Long-term debt maturities due in 2013	1.2
Deferred financing costs	3.6	2.6
Dividend Restrictions [Abstract]
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met	$ 31.8
Minimum calendar year average equity-to-total capitalization ratio authorized by state commission (in hundredths)	52.50%
Calendar year average equity-to-total capitalization ratio (in hundredths)	52.60%