Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 49	$ (65)	$ 116	$ (33)
Private Equity Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Liquidations term			10 years
Alabama Power [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	26	(39)	$ 66	(19)
Georgia Power [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 23	$ (26)	$ 50	$ (14)
Southern Power [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent Payment Obligations, Payment Period			10 years