Summary of Significant Accounting Policies - Nuclear Decommissiong (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	$ 1,832	$ 1,606
Proceeds from sale of securities held in external trust funds	800	1,200	$ 1,400
Increase (decrease) in fair value of securities related to nuclear decommissioning	233	114	11
Plant Farley
Decommissioning
Total site study costs	1,442
Plant Farley | Spent fuel management
Decommissioning
Total site study costs	0
Plant Hatch
Decommissioning
Total site study costs	902
Plant Vogtle (nuclear) Units 1 and 2
Decommissioning
Total site study costs	804
Equity Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	1,100	878
Debt Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	725	685
Other Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	47	41
Unrealized Loss | Securities Held in Funds
Public Utility, Property, Plant and Equipment [Line Items]
Increase (decrease) in fair value of securities related to nuclear decommissioning		(48)
Unrealized Gain | Securities Held in Funds
Public Utility, Property, Plant and Equipment [Line Items]
Increase (decrease) in fair value of securities related to nuclear decommissioning	181		83
ALABAMA POWER CO
Public Utility, Property, Plant and Equipment [Line Items]
Decommissioning fund investments net of foreign currency	902	790
Nuclear decommissioning trusts, at fair value	903	792
Proceeds from sale of securities held in external trust funds	237	351	438
Increase (decrease) in fair value of securities related to nuclear decommissioning	125	76	8
Accumulated Provisions for Decommissioning
Accumulated Provisions for Decommissioning	920	809
Decommissioning
Total site study costs	$ 1,442
Significant assumption of inflation rate used to determine the costs for rate making (as percent)	4.50%
Significant assumption of trust earnings rate used to determine the costs for rate making (as percent)	7.00%
ALABAMA POWER CO | Accumulated provisions for the external decommissioning trust funds
Accumulated Provisions for Decommissioning
Accumulated Provisions for Decommissioning		790
ALABAMA POWER CO | Accumulated Provisions for Decommissioning Internal Reserves
Accumulated Provisions for Decommissioning
Accumulated Provisions for Decommissioning	$ 18	19
ALABAMA POWER CO | Plant Farley | Plant Farley
Decommissioning
Beginning Year	2037
Completion Year	2076
ALABAMA POWER CO | Plant Farley | Accumulated provisions for the external decommissioning trust funds
Accumulated Provisions for Decommissioning
Accumulated Provisions for Decommissioning	$ 902	790
ALABAMA POWER CO | Plant Farley | Radiated structures
Decommissioning
Total site study costs	1,362
ALABAMA POWER CO | Plant Farley | Non-radiated structures
Decommissioning
Total site study costs	80
ALABAMA POWER CO | Equity Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	644	552
ALABAMA POWER CO | Debt Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	223	208
ALABAMA POWER CO | Other Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	35	30
ALABAMA POWER CO | Securities Held in Funds
Public Utility, Property, Plant and Equipment [Line Items]
Increase (decrease) in fair value of securities related to nuclear decommissioning		(34)	57
ALABAMA POWER CO | Unrealized Gain | Securities Held in Funds
Public Utility, Property, Plant and Equipment [Line Items]
Increase (decrease) in fair value of securities related to nuclear decommissioning	98
GEORGIA POWER CO
Public Utility, Property, Plant and Equipment [Line Items]
Environmental regulatory assets	49	35
Fair market value of fund's securities on loan under the Funds' managers' securities lending program	76	56
Fair value of collateral received	77	58
Decommissioning fund investments net of foreign currency	929	814
Nuclear decommissioning trusts, at fair value	929	814
Increase (decrease) in fair value of securities related to nuclear decommissioning	$ 108	38	3
Decommissioning
Significant assumption of inflation rate used to determine the costs for rate making (as percent)	2.40%
Significant assumption of trust earnings rate used to determine the costs for rate making (as percent)	4.40%
GEORGIA POWER CO | Plant Hatch
Decommissioning
Beginning Year	2034
Completion Year	2075
Total site study costs	$ 902
Amount expensed for rate making purpose				$ 4
GEORGIA POWER CO | Plant Hatch | Accumulated provisions for the external decommissioning trust funds
Accumulated Provisions for Decommissioning
Accumulated Provisions for Decommissioning	583	511
GEORGIA POWER CO | Plant Hatch | Radiated structures
Decommissioning
Total site study costs	678
GEORGIA POWER CO | Plant Hatch | Spent fuel management
Decommissioning
Total site study costs	160
GEORGIA POWER CO | Plant Hatch | Non-radiated structures
Decommissioning
Total site study costs	$ 64
GEORGIA POWER CO | Plant Vogtle (nuclear) Units 1 and 2
Decommissioning
Beginning Year	2047
Completion Year	2079
Total site study costs	$ 804
GEORGIA POWER CO | Plant Vogtle (nuclear) Units 1 and 2 | Accumulated provisions for the external decommissioning trust funds
Accumulated Provisions for Decommissioning
Accumulated Provisions for Decommissioning	346	303
GEORGIA POWER CO | Plant Vogtle (nuclear) Units 1 and 2 | Radiated structures
Decommissioning
Total site study costs	568
GEORGIA POWER CO | Plant Vogtle (nuclear) Units 1 and 2 | Spent fuel management
Decommissioning
Total site study costs	147
GEORGIA POWER CO | Plant Vogtle (nuclear) Units 1 and 2 | Non-radiated structures
Decommissioning
Total site study costs	89
GEORGIA POWER CO | Plant Vogtle
Decommissioning
Amount expensed for rate making purpose				$ 2
GEORGIA POWER CO | Equity Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	415	326
GEORGIA POWER CO | Debt Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	502	477
GEORGIA POWER CO | Other Securities
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	12	11
GEORGIA POWER CO | Securities Investment
Public Utility, Property, Plant and Equipment [Line Items]
Nuclear decommissioning trusts, at fair value	568	803			$ 980
GEORGIA POWER CO | Securities Held in Funds
Public Utility, Property, Plant and Equipment [Line Items]
Increase (decrease) in fair value of securities related to nuclear decommissioning	83		$ 26
GEORGIA POWER CO | Unrealized Gain (Loss or Write-down)
Public Utility, Property, Plant and Equipment [Line Items]
Increase (decrease) in fair value of securities related to nuclear decommissioning		(14)
Other regulatory assets current | GEORGIA POWER CO
Public Utility, Property, Plant and Equipment [Line Items]
Environmental regulatory assets	2	2
Other regulatory assets deferred | GEORGIA POWER CO
Public Utility, Property, Plant and Equipment [Line Items]
Environmental regulatory assets	$ 47	$ 33