Fair Value Measurements - Fair Value, Nature and Risk of Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Foreign equity fund [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 121,000,000	$ 131,000,000
Unfunded Commitments	0	0
Redemption Frequency	Monthly	Monthly
Redemption Notice Period	5 days	5 days
Corporate bonds - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value		8,000,000
Unfunded Commitments	0	0
Redemption Frequency		Daily
Redemption Notice Period		Not applicable
Equity - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	63,000,000	65,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily/Monthly	Daily/Monthly
Redemption Notice Period	Daily/7 days	Daily/7 days
Debt - Commingled Funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	15,000,000
Unfunded Commitments	0
Redemption Frequency	Daily
Redemption Notice Period	5 days
Other - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	8,000,000	24,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable
Other - money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	11,000,000
Unfunded Commitments	0
Redemption Frequency	Daily
Redemption Notice Period	Not applicable
Trust-owned life insurance [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	115,000,000	110,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	15 days	15 days
Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	397,000,000	491,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable
Maximum [Member] | Foreign equity fund [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Redemption Notice Period	5 days	5 days
Maximum [Member] | Equity - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Redemption Notice Period	7 days	7 days
Maximum [Member] | Debt - Commingled Funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Redemption Notice Period	5 days
Maximum [Member] | Trust-owned life insurance [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Redemption Notice Period	15 days	15 days
Alabama Power [Member] | Equity - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	63,000,000	65,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily/Monthly	Daily/Monthly
Redemption Notice Period	Daily/7 days	Daily/7 days
Redemption Notice Period	7 days	7 days
Alabama Power [Member] | Debt - Commingled Funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	15,000,000
Unfunded Commitments	0
Redemption Frequency	Daily
Redemption Notice Period	5 days
Redemption Notice Period	5 days
Alabama Power [Member] | Trust-owned life insurance [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	115,000,000	110,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	15 days	15 days
Redemption Notice Period	15 days	15 days
Alabama Power [Member] | Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	162,000,000
Cash and Cash Equivalents, Fair Value Disclosure		236,000,000
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable
Georgia Power [Member] | Foreign equity fund [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	121,000,000	131,000,000
Unfunded Commitments	0	0
Redemption Frequency	Monthly	Daily
Redemption Notice Period	5 days	5 days
Georgia Power [Member] | Corporate bonds - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value		8,000,000
Unfunded Commitments	0	0
Redemption Frequency		Daily
Redemption Notice Period		Not applicable
Georgia Power [Member] | Other - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	8,000,000	24,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable
Georgia Power [Member] | Other - money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	11,000,000
Unfunded Commitments	0
Redemption Frequency	Daily
Redemption Notice Period	Not applicable
Georgia Power [Member] | Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Unfunded Commitments		0
Georgia Power [Member] | Maximum [Member] | Foreign equity fund [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Redemption Notice Period	5 days	5 days
Gulf Power [Member] | Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	18,032,000	15,929,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable
Mississippi Power [Member] | Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	114,900,000	125,000,000
Unfunded Commitments	0	0
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable
Southern Power [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	18,000,000	68,000,000
Unfunded Commitments	$ 0	$ 0
Redemption Frequency	Daily	Daily
Redemption Notice Period	Not applicable	Not applicable