Summary of Significant Accounting Policies - Customer Revenues (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Concentration Risk [Line Items]
Maximum Revenue from Single Customer or Industry	10.00%
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	15.80%	10.10%	11.80%
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	10.70%	9.70%	10.70%
Customer Concentration Risk [Member] | Southern Power [Member] | Duke Energy Corporation [Member] | Sales Revenue, Goods, Net [Member]
Concentration Risk [Line Items]
Maximum Revenue from Single Customer or Industry	8.20%	9.10%	10.30%