REGULATORY MATTERS - GAS Rate Proceedings and Infrastructure Replacement Programs and Capital Projects (Details) - USD ($) $ in Millions						1 Months Ended			12 Months Ended
	Feb. 16, 2021	Jan. 14, 2021	Jul. 01, 2020	Jun. 01, 2020	May 05, 2018	Dec. 31, 2019	Oct. 31, 2019	Jan. 31, 2018	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2018	Jul. 31, 2018
Public Utilities, General Disclosures [Line Items]
Net regulatory assets						$ 3,838.0			$ 5,493.0
Nicor Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount					$ (44.0)		$ 168.0	$ 137.0
Authorized ROE							9.73%	9.80%	9.73%
Public utilities, approved equity capital structure, percentage							54.20%
Nicor Gas | COVID-19
Public Utilities, General Disclosures [Line Items]
Net regulatory assets									$ 9.0
Nicor Gas | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Requested rate increase, amount		$ 293.0
Requested rate increase, amount, recovery of investments		$ 94.0
Requested ROE		10.35%
Requested equity ratio		54.50%
Nicor Gas | Investing In Illinois
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase amount recovery of investments							$ 65.0	$ 93.0
Atlanta Gas Light
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount						$ 65.0				$ (16.0)
Authorized ROE						10.25%			10.25%
Public utilities, approved equity capital structure, percentage						56.00%
Requested rate increase, amount			$ 37.6
Requested rate increase, projected test year, term			12 months
Contract with customer, liability, bill credits												$ 8.0	$ 8.0
Approved rate increase, percent										4.00%
Regulatory equity ratio										55.00%
Public utilities, maximum base rate percentage			5.00%						5.00%
Atlanta Gas Light | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Integrated capacity delivery plan, interstate and intrastate capacity asset requirements, term	10 years
Integrated capacity delivery plan, interstate and intrastate capacity asset requirements, detailed plan term	3 years
Integrated capacity delivery plan, capital budgets and related operations and maintenance spending, term	10 years
Atlanta Gas Light | Minimum
Public Utilities, General Disclosures [Line Items]
Authorized ROE						10.05%				10.55%
Atlanta Gas Light | Maximum
Public Utilities, General Disclosures [Line Items]
Authorized ROE						10.45%				10.95%
Virginia Natural Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount										$ (14.0)
Authorized ROE									9.50%		9.50%
Requested rate increase, amount				$ 49.6
Requested rate increase, projected test year, term				12 months
Requested ROE				10.35%
Requested equity ratio				54.00%
Contract with customer, liability, bill credits										$ 14.0
Virginia Natural Gas | Minimum
Public Utilities, General Disclosures [Line Items]
Authorized ROE											9.00%
Virginia Natural Gas | Maximum
Public Utilities, General Disclosures [Line Items]
Authorized ROE											10.00%