Borrowings and Other Financing Instruments, Letters of Credit (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Amount outstanding at period end	37,000	$ 84,000
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	30,000	$ 25,500
Letter of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)	1 year