Borrowings and Other Financing Instruments, Letters of Credit (Details) (USD $) In Thousands, unless otherwise specified					9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Letter of Credit	Dec. 31, 2013 Letter of Credit	Sep. 30, 2014 Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)					1 year
Amount outstanding at period end	$ 253,000	$ 0	$ 6,500	$ 6,400