Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) (USD $)	1 Months Ended		12 Months Ended
	Aug. 31, 2013	Jun. 30, 2012	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Three			$ 200,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five			250,000,000
Deferred Finance Costs, Noncurrent, Net			10,300,000	9,700,000
Equity to total capitalization ratio (excluding short-term debt), low end of range			45.00%
Equity to total capitalization ratio (excluding short-term debt), high end of range			55.00%
Equity to total capitalization ratio (excluding short-term debt)			53.20%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions			359,000,000
First Mortgage Bonds | Series Due Aug. 15, 2041
Debt Instrument [Line Items]
Face amount	100,000,000	100,000,000
Debt instrument, interest rate stated percentage (in hundredths)	4.50%	4.50%	4.50%
Maturity date	Aug. 15, 2041	Aug. 15, 2041	Aug. 15, 2041
Principal outstanding			$ 400,000,000	$ 300,000,000