Summary of Significant Accounting Policies - Affiliate Transactions (Details) (Southern Power [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	$ (12.4)	$ (17.6)
Deferred Capacity Revenues Affiliated [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	(15.3)	(15.3)
Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	0	(4.2)
Other Deferred Charges and assets - Affiliated [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	$ 2.9	$ 1.9