Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 55	$ 47	$ 118	$ 67
Private Equity
Fair Value, Option, Quantitative Disclosures [Line Items]
Liquidations term			10 years
ALABAMA POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	28	29	$ 62	40
GEORGIA POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Increase (decrease) in fair value of funds	$ 27	$ 18	$ 56	$ 27
Minimum | SOUTHERN POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent payment obligations, payment period			10 years
Maximum | SOUTHERN POWER CO
Fair Value, Option, Quantitative Disclosures [Line Items]
Contingent payment obligations, payment period			30 years