Borrowings and Other Financing Instruments, Issuances of Common Stock (Details) (At-the-Market Program, USD $)	1 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended
	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013	Apr. 30, 2014 Subsequent Event
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)		2,100,000	7,700,000
Net cash proceeds from issuance of common stock		62,000,000	223,000,000
Fees and commissions from issuance of common stock		1,000,000	3,000,000
Shares of common stock sold and recorded on a settlement date basis (in shares)				500,000
Net cash proceeds from sale of common stock recorded on a settlement date basis				16,000,000
Cumulative fees and commissions from common stock sold and recorded on settlement date basis				$ 200,000