Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 63	$ 20
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	$ 34	11
Georgia Power [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 29	$ 9
Minimum [Member] | Southern Power [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent payment obligations, payment period	10 years
Maximum [Member] | Southern Power [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent payment obligations, payment period	30 years