Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) (USD $)	1 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2014		Dec. 31, 2013
Long-Term Borrowings [Line Items]
Long-term Line of Credit				$ 0
Deferred Finance Costs, Noncurrent, Net		26,500,000		37,000,000
First Mortgage Bonds | Series Due March 15, 2044
Long-Term Borrowings [Line Items]
Face amount	300,000,000
Interest rate, stated percentage (in hundredths)	4.30%	4.30%
Maturity date	Mar. 15, 2044	Mar. 15, 2044
Long-term Debt, Gross		300,000,000		0
First Mortgage Bonds | Series Due Sept. 1, 2017
Long-Term Borrowings [Line Items]
Interest rate, stated percentage (in hundredths)		4.375%
Maturity date		Sep. 01, 2017
Long-term Debt, Gross		129,500,000	[1]	129,500,000	[1]
First Mortgage Bonds | Series Due Aug. 1, 2018
Long-Term Borrowings [Line Items]
Interest rate, stated percentage (in hundredths)		5.80%
Maturity date		Aug. 01, 2018
Long-term Debt, Gross		300,000,000		300,000,000
First Mortgage Bonds | Series Due June 1, 2019
Long-Term Borrowings [Line Items]
Interest rate, stated percentage (in hundredths)		5.125%
Maturity date		Jun. 01, 2019
Long-term Debt, Gross		400,000,000		400,000,000
First Mortgage Bonds | Series Due March 15, 2023
Long-Term Borrowings [Line Items]
Interest rate, stated percentage (in hundredths)	2.50%	2.50%
Maturity date	Mar. 15, 2023	Mar. 15, 2023
Long-term Debt, Gross		250,000,000		250,000,000
First Mortgage Bonds | Series Due March 15, 2043
Long-Term Borrowings [Line Items]
Interest rate, stated percentage (in hundredths)	3.95%	3.95%
Maturity date	Mar. 15, 2043	Mar. 15, 2043
Long-term Debt, Gross		$ 250,000,000		$ 250,000,000

[1] Pollution control financing.