Borrowings and Other Financing Instruments, Letters of Credit (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Letters of Credit [Abstract]
Terms of letters of credit (in years)	1 year
Letters of credit outstanding under credit facilities		$ 4.0	$ 4.9