Borrowings and Other Financing Instruments, Issuances of Common Stock (Details) (At-the-Market Program, USD $) Share data in Thousands, unless otherwise specified	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2013	Jun. 30, 2014	Dec. 31, 2013
At-the-Market Program
Issuances of Common Stock [Abstract]
Maximum aggregate gross sales price of common stock that can be offered and sold	$ 400,000,000
Issuances of common stock (in shares)		5,700	7,700
Net cash proceeds from issuance of common stock		172,700,000	222,700,000
Fees and commissions from issuance of common stock		$ 1,900,000	$ 2,700,000