Borrowings and Other Financing Instruments, Letters of Credit (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Amount outstanding at period end	142,000	$ 382,000
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	7,000	$ 6,000
Letter of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)	1 year