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REGULATORY MATTERS - GAS Infrastructure Replacement Programs and Capital Projects (Details) $ in Millions	12 Months Ended					21 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) mi	Dec. 31, 2018 USD ($)	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2019 USD ($)	Dec. 15, 2020 mi
Public Utilities, General Disclosures [Line Items]
Capital expenditures related to qualifying projects	$ 7,441	$ 7,555	$ 8,001
Southern Company Gas
Public Utilities, General Disclosures [Line Items]
Capital expenditures related to qualifying projects	1,471	1,408	1,388
Nicor Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase, recovery of investment amount						$ 65
Capital expenditures related to qualifying projects		$ 396	$ 409
Nicor Gas \| Regulatory Infrastructure Program
Public Utilities, General Disclosures [Line Items]
Program duration period				9 years
Nicor Gas \| Maximum
Public Utilities, General Disclosures [Line Items]
Infrastructure investment, annual customer rate increase				4.00%
Infrastructure investment, annual customer rate increase in any given year				5.50%
Virginia Natural Gas
Public Utilities, General Disclosures [Line Items]
Header improvement project, scope of project (in miles) \| mi		24.1					9.5
Virginia Natural Gas \| SAVE
Public Utilities, General Disclosures [Line Items]
Program duration period					6 years
Approved infrastructure replacement program, approved investment amount, current fiscal year	365
Infrastructure replacement program, approved investment variance amount	5
Virginia Natural Gas \| SAVE \| Year 2018
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	35
Virginia Natural Gas \| SAVE \| Year 2019
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	40
Virginia Natural Gas \| SAVE \| Year 2020
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	50
Virginia Natural Gas \| SAVE \| Year 2021
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	60
Virginia Natural Gas \| SAVE \| Year 2022
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	70
Virginia Natural Gas \| SAVE \| Year 2023
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	70
Virginia Natural Gas \| SAVE \| Year 2024
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	70
Atlanta Gas Light
Public Utilities, General Disclosures [Line Items]
Annual proceeds from strategic economic development projects	15
Atlanta Gas Light \| True Up Recovery
Public Utilities, General Disclosures [Line Items]
Gain contingency, unrecorded amount	113
Atlanta Gas Light \| True-Up Recovery, Unrecognized Equity
Public Utilities, General Disclosures [Line Items]
Gain contingency, unrecorded amount	59
Operating Segments \| Gas distribution operations \| Southern Company Gas
Public Utilities, General Disclosures [Line Items]
Gross property additions	$ 1,500