Borrowings and Other Financing Instruments, Issuances of Common Stock (Details) (USD $) Share data in Thousands, unless otherwise specified	9 Months Ended			12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 At-the-Market Program	Dec. 31, 2013 At-the-Market Program	Sep. 30, 2014 At-the-Market Program Common Stock	Dec. 31, 2013 At-the-Market Program Common Stock
Class of Stock [Line Items]
Issuances of common stock (in shares)					5,700	7,700
Proceeds from issuance of common stock	$ 178,639,000	$ 229,420,000	$ 172,700,000	$ 222,700,000
Fees and commissions from issuance of common stock			$ 1,900,000	$ 2,700,000