Acquisitions and Dispositions - Southern Company Gas (Details) $ in Millions				3 Months Ended			6 Months Ended
	Jul. 29, 2018 USD ($)	Jul. 01, 2018 USD ($) utility	Jun. 04, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Aug. 07, 2018 utility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment							$ 42
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
Gain (loss) on disposition of business				(36)		$ 0	(36)	$ 0
Pivotal Home Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations | SOUTHERN Co GAS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash purchase price on sale of business			$ 358
Working capital adjustment			6
Gain (loss) on disposition of business, after tax			(76)
Tax on gain (loss) on disposition of business			$ 40
Goodwill impairment					$ 42		42
Florida City Gas | Disposal Group, Disposed of by Sale, Not Discontinued Operations | SOUTHERN Co GAS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term debt				$ 3			$ 3
Subsequent Event | Disposal Group, Disposed of by Sale, Not Discontinued Operations | SOUTHERN Co GAS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of natural gas distribution utilities disposed | utility									3
Subsequent Event | Elizabethtown Gas and Elkton Gas | Disposal Group, Disposed of by Sale, Not Discontinued Operations | SOUTHERN Co 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
Working capital adjustment		40
Gain (loss) on disposition of business, after tax		$ (12)
Number of natural gas distribution utilities disposed | utility		2
Gain (loss) on disposition of business		$ 235
Subsequent Event | Florida City Gas | Disposal Group, Disposed of by Sale, Not Discontinued Operations | SOUTHERN Co 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	4
Gain (loss) on disposition of business	126
Additional cash amount	$ 60