REGULATORY MATTERS - GAS Regulatory Matters (Details) - USD ($) $ in Millions											1 Months Ended			3 Months Ended		12 Months Ended					21 Months Ended
	Jan. 31, 2020	Dec. 19, 2019	Oct. 02, 2019	Dec. 31, 2018	Dec. 17, 2018	May 15, 2018	May 05, 2018	Feb. 23, 2018	Dec. 21, 2017	Oct. 10, 2017	Oct. 31, 2018	Jul. 31, 2018	Jan. 31, 2018	Jun. 30, 2020	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2019
Schedule of Regulatory Assets and Liabilities [Line Items]
Contract with customer, liability, bill credits																$ 50
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), recovery of investment amount																					$ 65
Public utilities, approved rate increase (decrease), amount			$ 168				$ (44)						$ 137
Public utilities, approved return on equity			9.73%										9.80%
Public utilities, approved equity capital structure, percentage			54.20%
Nicor Gas | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Infrastructure investment, annual customer rate increase																			4.00%
Infrastructure investment, annual customer rate increase in any given year																			5.50%
Nicor Gas | Regulatory Infrastructure Program
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period																		9 years
Nicor Gas | Investing In Illinois
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease) amount recovery of investments			$ 65										$ 93
Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount					$ (14)				$ 34
Public utilities, approved return on equity									9.50%
Contract with customer, liability, bill credits				$ 14
Virginia Natural Gas | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity									9.00%
Virginia Natural Gas | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity									10.00%
Virginia Natural Gas | SAVE
Schedule of Regulatory Assets and Liabilities [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
Public utilities, approved rate increase (decrease) amount recovery of investments									$ 13
Virginia Natural Gas | Year 2018 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																35
Virginia Natural Gas | Year 2019 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																40
Virginia Natural Gas | Year 2020 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																50
Virginia Natural Gas | Year 2021 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																60
Virginia Natural Gas | Year 2022 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																70
Virginia Natural Gas | Year 2023 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																70
Virginia Natural Gas | Year 2024 | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																$ 70
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount		$ 65						$ 16
Public utilities, approved return on equity		10.25%
Public utilities, approved equity capital structure, percentage		56.00%
Annual proceeds from strategic economic development projects										$ 15
Public utilities, maximum base rate percentage																5.00%
Regulatory equity ratio						55.00%
Contract with customer, liability, bill credits											$ 8	$ 8
Public utilities approved rate increase (decrease)						4.00%
Atlanta Gas Light | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity		10.05%															10.55%
Atlanta Gas Light | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity		10.45%															10.95%
Atlanta Gas Light | Favorable Regulatory Action
Schedule of Regulatory Assets and Liabilities [Line Items]
Proceeds from legal settlements															$ 7		$ 20
Atlanta Gas Light | True Up Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Gain contingency, unrecorded amount																$ 135
Atlanta Gas Light | True-Up Recovery, Unrecognized Equity
Schedule of Regulatory Assets and Liabilities [Line Items]
Gain contingency, unrecorded amount																70
Southern Company Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered fuel cost				$ 15												$ 74	$ 15
Subsequent Event | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Comprehensive capacity supply and related infrastructure delivery plan, term	10 years
Updated comprehensive capacity supply and related infrastructure delivery plan, at least once every third year, term	10 years
Scenario, Forecast | Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, estimated project costs														$ 346