REGULATORY MATTERS - GAS Rate Proceedings and Deferral of Incremental COVID-19 Costs (Details) - USD ($) $ in Millions								1 Months Ended	12 Months Ended
	Jan. 01, 2022	Nov. 18, 2021	Sep. 14, 2021	Apr. 28, 2021	Feb. 16, 2021	Nov. 01, 2020	Jul. 01, 2020	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2019	Dec. 31, 2020	Jul. 31, 2020
Public Utilities, General Disclosures [Line Items]
Net regulatory assets									$ 4,711.0		$ 5,493.0
Nicor Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount		$ 240.0								$ 168.0
Authorized ROE		9.75%							9.75%	9.73%
Public utilities, approved equity capital structure, percentage		54.50%								54.20%
Nicor Gas | COVID-19
Public Utilities, General Disclosures [Line Items]
Net regulatory assets									$ 5.0		$ 9.0
Nicor Gas | Investing In Illinois
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase amount recovery of investments		$ 94.0								$ 65.0
Atlanta Gas Light
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount		$ (5.0)						$ 65.0
Authorized ROE								10.25%	10.25%
Public utilities, approved equity capital structure, percentage								56.00%
Public utilities, maximum base rate percentage									5.00%			5.00%
Requested rate increase (decrease), amount							$ 37.6
Requested rate increase, projected test year, term							12 months
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	10 years
Approved rate decrease, percent		10.00%
Integrated capacity delivery plan, capital forecast plan, years one through three		$ 1,700.0
Atlanta Gas Light | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount	$ (43.0)
Atlanta Gas Light | Minimum
Public Utilities, General Disclosures [Line Items]
Authorized ROE								10.05%
Integrated capacity delivery plan, capital forecast plan, years one through ten				$ 500.0
Atlanta Gas Light | Maximum
Public Utilities, General Disclosures [Line Items]
Authorized ROE								10.45%
Integrated capacity delivery plan, capital forecast plan, years one through ten				$ 600.0
Virginia Natural Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount			$ 43.0
Public utilities, approved rate increase amount recovery of investments			$ 14.0
Authorized ROE			9.50%						9.50%
Public utilities, approved equity capital structure, percentage			51.90%
Requested rate increase (decrease), amount						$ 50.0