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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 |  |  |  |  |  |

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