Acquisitions and Divestitures - Textual (Details) (Southern Power [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2010	Dec. 31, 2012 MW	Sep. 28, 2012 Spectrum Nevada Solar Llc [Member] MW	Jun. 29, 2012 Apex Nevada Solar Llc [Member] MW	Oct. 16, 2012 Granville Solar Llc [Member] MW	Dec. 31, 2012 Southern Renewable Energy, Inc. [Member]	Mar. 15, 2011 Plant Cimarron [Member] Southern Renewable Energy, Inc. [Member] MW
Business Acquisition [Line Items]
Power of Solar Polycrystalline Silicon Facility			30	20	2.5
Life Output Of Plant			25 years	25 years	20 years
Beginning Year of Output of Constructed Plant			2013	2012	2012
Effect of acquisition on net income	$ 1.3
Plant capacity (in megawatts)		659					30
Ending Year of Output of Constructed Plant						2035
Cash consideration paid for acquisition			17.6	102.0	10.4		100.0
Business Acquisition Cost of Acquired Entity Purchase Consideration Cash Paid			$ 99.9