Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) (First Mortgage Bonds, USD $)	1 Months Ended
	Mar. 31, 2013
Series Due March 15, 2023
Long-Term Borrowings [Abstract]
Face amount	$ 250,000,000
Interest rate, stated percentage (in hundredths)	2.50%
Maturity date	Mar. 15, 2023
Series Due March 15, 2043
Long-Term Borrowings [Abstract]
Face amount	$ 250,000,000
Interest rate, stated percentage (in hundredths)	3.95%
Maturity date	Mar. 15, 2043