Borrowings and Other Financing Instruments, Issuances of Common Stock (Details) - USD ($) $ in Thousands, shares in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Proceeds from issuance of common stock	$ 7,011	$ 180,798	$ 231,767
At-the-Market Program
Class of Stock [Line Items]
Proceeds from issuance of common stock		172,700
Fees and commissions from issuance of common stock		$ 1,900
At-the-Market Program | Common stock
Class of Stock [Line Items]
Issuances of common stock (in shares)		5.7