Summary of Significant Accounting Policies - Customer Revenues (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Maximum Revenue from Single Customer or Industry	10.00%
Customer Concentration Risk [Member] | Southern Power [Member] | Florida Power and Light [Member] | Sales Revenue, Goods, Net [Member]
Concentration Risk [Line Items]
Maximum Revenue from Single Customer or Industry	10.10%	11.80%	12.80%
Customer Concentration Risk [Member] | Southern Power [Member] | Georgia Power [Member] | Sales Revenue, Goods, Net [Member]
Concentration Risk [Line Items]
Maximum Revenue from Single Customer or Industry	9.70%	10.70%	12.50%
Customer Concentration Risk [Member] | Southern Power [Member] | Progress Energy Florida [Member] | Sales Revenue, Goods, Net [Member]
Concentration Risk [Line Items]
Maximum Revenue from Single Customer or Industry	9.10%	10.30%	5.90%