Acquisitions and Dispositions - Southern Company Gas (Details) $ in Millions				3 Months Ended			9 Months Ended
	Jul. 29, 2018 USD ($)	Jul. 01, 2018 USD ($) utility	Jun. 04, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jul. 31, 2018 utility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment							$ 42
Gain on disposition of business							324	$ 22
SOUTHERN Co GAS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment				$ 0	$ 42	$ 0	42	$ 0
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of natural gas distribution utilities disposed | utility									3
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Pivotal Home Solutions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business			$ 365
Gain (loss) on disposition of business, after tax			(73)
Tax on gain (loss) on disposition of business			$ 39
Goodwill impairment					$ 42		$ 42
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Elizabethtown Gas and Elkton Gas
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business		$ 1,700
Gain (loss) on disposition of business, after tax		$ 18
Number of natural gas distribution utilities disposed | utility		2
Working capital adjustment		$ 40
Gain on disposition of business		$ 230
SOUTHERN Co GAS | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Florida City Gas
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business	$ 530
Gain (loss) on disposition of business, after tax	20
Gain on disposition of business	121
Long-term debt	3
Additional cash amount	$ 60