Industry Regulation - Additional Information (Details) MWh in Millions												1 Months Ended					12 Months Ended
	Dec. 31, 2020 USD ($) project	Sep. 23, 2020 project	Feb. 19, 2020 USD ($)	Jan. 21, 2020	Dec. 09, 2019 USD ($)	May 20, 2019	Jan. 18, 2019	Dec. 28, 2018 MWh project	Sep. 10, 2018 USD ($)	Jan. 01, 2017	Mar. 31, 2010 USD ($) MW	Aug. 31, 2019 MW	Oct. 31, 2018 agreement MW	Dec. 31, 2017	Mar. 31, 2017 order project	Dec. 31, 2016	Dec. 31, 2020 USD ($) company project	Apr. 30, 2019	Jun. 22, 2020	Apr. 27, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 31, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Number of networks supply companies | company																	8
Approved return on equity													9.88%
Equity ratio	3.00%						54.00%							50.00%			3.00%
Percentage of standard service customers with wholesale power supply agreements in place for the second half of 2021																	70.00%
Percentage of standard service customers with wholesale power supply agreements in place for the first half of 2022																	20.00%
Public utilities regulatory authority distribution rate							9.70%
Number of REV related orders issued | order															3
Number of energy storage projects | project															2
Regulatory assets	$ 2,882,000,000																$ 2,882,000,000				$ 2,861,000,000	$ 11,000,000
Number of average months used to set rate																	13 months
Restricted net assets	5,408,000,000																$ 5,408,000,000
Rate plan modification, overall bill impact limit, percent																			2.00%
Purchase obligation per year	$ 2,125,000,000																$ 2,125,000,000
DEEP
Public Utilities, General Disclosures [Line Items]
PPA capacity (in MWH) | MWh								12
Maine Public Utilities Commission (MPUC)
Public Utilities, General Disclosures [Line Items]
Number of projects | project		14
PURA
Public Utilities, General Disclosures [Line Items]
Equity ratio										50.00%
Percentage of return on equity (ROE)																9.10%	9.30%
New distribution rate schedule, period																3 years
Public utilities regulatory authority distribution rate										50.00%
Equity ratio, year one	54.00%																54.00%
Equity ratio, year two	54.50%																54.50%
Equity ratio, year three	55.00%																55.00%
Power Tax
Public Utilities, General Disclosures [Line Items]
Depreciation amortization period				32 years 6 months
Depreciation amortization period				32 years 6 months
Storm costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	$ 451,000,000																$ 451,000,000				272,000,000
Federal tax depreciation normalization adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory assets	148,000,000																148,000,000				$ 153,000,000
Central Maine Power
Public Utilities, General Disclosures [Line Items]
Distribution revenue requirement			$ 17,000,000
Annual distribution tariff increase percentage			7.00%
Distribution tariff rate increased based on ROE			9.25%
Distribution tariff rate increased based on equity capital			50.00%
ROE reduction			1.00%
Proposed distribution tariff rate decrease based on return on equity			8.25%
Customer service performance period			18 months
Benchmark amount of metering, billing and customer service practices audit costs (less than)																				$ 1,000,000
SCG
Public Utilities, General Disclosures [Line Items]
Equity ratio														52.00%
Percentage of return on equity (ROE)														9.25%
NYDPS
Public Utilities, General Disclosures [Line Items]
Equity ratio for earnings sharing						48.00%
Percentage of return on equity (ROE)						8.80%
United Illuminating Company (UI)
Public Utilities, General Disclosures [Line Items]
Maximum amount of commitment to purchase renewable energy credits (recs) from new facilities behind distribution customer meters	200,000,000																$ 200,000,000
Proposed off shore wind capacity (in MW) | MW													50
Period of purchase commitment																	21 years
Solicitation period																	6 years
Maximum annual commitment level obligation after year six	14,000,000																$ 14,000,000
Additional maximum annual commitment	$ 64,000,000																$ 64,000,000
Number of power purchase agreements | agreement													5
Number of projects | project								11
Vineyard Wind
Public Utilities, General Disclosures [Line Items]
Proposed off shore wind capacity (in MW) | MW												804
CMP
Public Utilities, General Disclosures [Line Items]
Period of purchase commitment	20 years										20 years
Wind farm capacity (in MW) | MW											60
Purchase obligation per year											$ 7,000,000
CMP | Dirigo Solar, LLC
Public Utilities, General Disclosures [Line Items]
Period of purchase commitment									20 years
Purchase obligation per year									$ 4,000,000
CMP | Maine Aqua Ventus I GP LLC
Public Utilities, General Disclosures [Line Items]
Period of purchase commitment					20 years
Purchase obligation per year					$ 12,000,000
CMP | Maine Public Utilities Commission (MPUC)
Public Utilities, General Disclosures [Line Items]
Number of projects | project	13
Number of projects executed | project	5																5
NYSEG Electric
Public Utilities, General Disclosures [Line Items]
Approved return on equity																	9.00%
Equity ratio	48.00%																48.00%
Equity ratio for earnings sharing	50.00%																50.00%
Recovery of deferred storm costs																	$ 262,000,000
NYSEG Electric | Storm costs, ten year recovery
Public Utilities, General Disclosures [Line Items]
Amortization of deferred storm cost																	123,000,000
NYSEG Electric | Storm costs, five year recovery
Public Utilities, General Disclosures [Line Items]
Amortization of deferred storm cost																	139,000,000
NYSEG Electric | Deferred Storm Cost Amortization 1 Year
Public Utilities, General Disclosures [Line Items]
Amortization of deferred storm cost																	$ 21,000,000
NYSEG Electric | Storm costs
Public Utilities, General Disclosures [Line Items]
Depreciation amortization period																	10 years
Depreciation amortization period																	10 years
NYSEG Electric | Regulatory Items Other Than Storm Costs
Public Utilities, General Disclosures [Line Items]
Depreciation amortization period																	5 years
Depreciation amortization period																	5 years
NYSEG Gas
Public Utilities, General Disclosures [Line Items]
Percentage of earnings customers received																	50.00%
Percentage of return on equity (ROE)																		9.75%
RG&E Electric
Public Utilities, General Disclosures [Line Items]
Percentage of earnings customers received																	75.00%
Percentage of return on equity (ROE)																		10.25%
RG&E Electric | Deferred Storm Cost Amortization 1 Year
Public Utilities, General Disclosures [Line Items]
Amortization of deferred storm cost																	$ 3,000,000
RG&E Gas
Public Utilities, General Disclosures [Line Items]
Percentage of earnings customers received																	90.00%
Percentage of return on equity (ROE)																		10.75%