Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) (USD $)			1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 First Mortgage Bonds Series Due March 15, 2023	Dec. 31, 2013 First Mortgage Bonds Series Due March 15, 2023	Mar. 31, 2013 First Mortgage Bonds Series Due March 15, 2043	Dec. 31, 2013 First Mortgage Bonds Series Due March 15, 2043	Sep. 30, 2012 First Mortgage Bonds Series Due Sept. 15, 2022	Dec. 31, 2013 First Mortgage Bonds Series Due Sept. 15, 2022	Sep. 30, 2012 First Mortgage Bonds Series Due Sept. 15, 2042	Dec. 31, 2013 First Mortgage Bonds Series Due Sept. 15, 2042	Oct. 31, 2012 First Mortgage Bonds Series Due Jan. 1, 2019
Long-Term Borrowings [Line Items]
Face amount			$ 250,000,000		$ 250,000,000		$ 300,000,000		$ 500,000,000
Interest rate, stated percentage (in hundredths)			2.50%	2.50%	3.95%	3.95%	2.25%	2.25%	3.60%	3.60%	5.10%
Extinguishment of Debt, Amount											48,750,000
Maturity date			Mar. 15, 2023	Mar. 15, 2023	Mar. 15, 2043	Mar. 15, 2043	Sep. 15, 2022	Sep. 15, 2022	Sep. 15, 2042	Sep. 15, 2042	Jan. 01, 2019
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	275,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	130,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	300,000,000
Deferred Finance Costs, Noncurrent, Net	$ 37,000,000	$ 23,000,000