Acquisitions and Dispositions - Acquisitions and Dispositions Table (Details) $ in Millions				1 Months Ended	6 Months Ended
	Jan. 26, 2018 MW	Jul. 31, 2017 MW	Jan. 06, 2017 MW	Jul. 31, 2018 MW	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Aug. 31, 2018	Dec. 31, 2017 USD ($)	Jul. 01, 2016 USD ($)	May 31, 2016 USD ($)
Purchase Price Allocation
Goodwill					$ 6,251	$ 5,315		$ 6,268
Southern Company Gas
Purchase Price Allocation
Current assets									$ 1,557
Property, plant, and equipment									10,108
Goodwill									5,967
Intangible assets									400
Regulatory assets									1,118
Other assets									229
Current liabilities									(2,201)
Other liabilities									(4,742)
Long-term debt									(4,261)
Noncontrolling interest									(174)
Total purchase price									8,001
PowerSecure International, Inc.
Purchase Price Allocation
Current assets										$ 172
Property, plant, and equipment										46
Goodwill										284
Intangible assets										106
Other assets										4
Current liabilities										(121)
Other liabilities										(14)
Long-term debt, including current portion										(48)
Total purchase price										$ 429
Southern Power
Purchase Price Allocation
Other assets								5
Restricted cash								16
CWIP								534
Accounts payable								(16)
Total purchase price								$ 539
Southern Power | Gaskell West 1
Acquisition Information, by Acquisition [Abstract]
Approximate Nameplate Capacity (MW) | MW	20
PPA Contract Period	20 years
Southern Power | Bethel
Acquisition Information, by Acquisition [Abstract]
Approximate Nameplate Capacity (MW) | MW			276
PPA Contract Period			12 years
Southern Power | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
Approximate Nameplate Capacity (MW) | MW		148		148
Minimum | Southern Power | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
PPA Contract Period		12 years		12 years
Maximum | Southern Power | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
PPA Contract Period		15 years		15 years
Gaskell West 1 | Class B Membership Interest | Southern Power | Gaskell West 1
Acquisition Information, by Acquisition [Abstract]
Ownership Percentage	100.00%
Cactus Flats | Southern Power | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
Ownership Percentage		100.00%
Cactus Flats | Class B Membership Interest | Southern Power | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
Ownership Percentage							100.00%
Bethel | Southern Power | Bethel
Acquisition Information, by Acquisition [Abstract]
Ownership Percentage			100.00%
Predecessor | Southern Company Gas
Purchase Price Allocation
Current assets									1,474
Property, plant, and equipment									10,148
Goodwill									1,813
Intangible assets									101
Regulatory assets									679
Other assets									273
Current liabilities									(2,205)
Other liabilities									(4,600)
Long-term debt									(3,709)
Noncontrolling interest									(41)
Total purchase price									$ 3,933
Change in Basis
Current assets					83
Property, plant, and equipment					(40)
Goodwill					4,154
Other intangible assets					299
Regulatory assets					439
Other assets					(44)
Current liabilities					4
Other liabilities					(142)
Long-term debt					(552)
Contingently redeemable noncontrolling interest					(133)
Total purchase price/equity					$ 4,068