Acquisitions, Development and Divestitures - Development (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended				4 Months Ended	6 Months Ended	9 Months Ended		21 Months Ended	25 Months Ended
	May 25, 2022	Jun. 30, 2022	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Jun. 30, 2022	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Jun. 30, 2024	Dec. 31, 2021
Business Acquisition [Line Items]
Business Combination, Integration Related Costs									$ 144.0	$ 10.0
Aggregate enterprise consideration								$ 5,300.0
Less: fair value of assumed long-term debt outstanding, net		$ 1,500.0			$ 1,500.0			1,500.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, AR Reserve	$ 60.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables, Gross	255.0
Business Combination, Transaction Costs Expense									18.0
Goodwill, Acquired During Period									1,579.0
Operating Revenues Pro Forma									6,322.0	5,435.0
Income (Loss) from Discontinued Operations (net of income taxes)				$ 0.0		$ (2.0)			0.0	(1,490.0)
Net Income (Loss) Pro Forma									630.0	(15.0)
Transition Service Agreement Cost				49.0					67.0
Goodwill				2,241.0			$ 2,241.0		2,241.0		$ 2,241.0		$ 716.0
Operating Revenues from external customers				2,134.0		1,512.0			5,612.0	4,298.0
Net income (loss)				174.0		207.0			566.0	(1,614.0)
Indemnification for Safari sale/leaseback				25.0
Guarantee Maximum Exposure for Safari Indemnification				151.0
Guarantee Maximum Exposure PPAs				55.0
Maximum Aggregate Liability for Safari Indemnification				140.0			140.0		140.0		140.0
Narragansett Electric
Business Acquisition [Line Items]
Total cash consideration								3,800.0
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | Safari Holdings LLC, Consolidated
Business Acquisition [Line Items]
Goodwill				53.0			53.0		53.0		53.0
Gain (Loss) on Disposition of Business				67.0
gain (loss) on disposition of business, net of tax				50.0
Subsequent Event [Member] | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | Safari Holdings LLC, Consolidated
Business Acquisition [Line Items]
Sales proceeds, net of realized foreign currency hedge losses			$ 120.0
Transaction Costs | Additional Costs Related to Purchase
Business Acquisition [Line Items]
Business Combination, Separately Recognized Transactions, Expenses and Losses Recognized											27.0
Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses | Transaction Costs | Additional Costs Related to Purchase
Business Acquisition [Line Items]
Business Combination, Separately Recognized Transactions, Expenses and Losses Recognized									18.0
Rhode Island Regulated
Business Acquisition [Line Items]
Goodwill				725.0			725.0		725.0		725.0
Operating Revenues from external customers				384.0		0.0			512.0	0.0
Net income (loss)				(26.0)		0.0			(55.0)	0.0
Corporate and Other [Member]
Business Acquisition [Line Items]
Goodwill, Purchase Accounting Adjustments									854.0
Operating Revenues from external customers				7.0		6.0			19.0	24.0
Net income (loss)				(96.0)		$ (76.0)			(223.0)	$ (848.0)
Corporate and Other [Member] | Adjustments identified in the third quarter
Business Acquisition [Line Items]
Goodwill, Purchase Accounting Adjustments				2.0
Rhode Island Energy
Business Acquisition [Line Items]
Business Combination, Regulatory Assets	75.0
Public Utilities, Customer Bill Credit Agreement, Total	50.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables	195.0
RICCREF Contribution Expense					2.5
Business Acquisition, Unbilled Revenues	54.0
Total Current Assets	630.0
Business Combination, Price Risk Management Assets	99.0
Property, Plant and Equipment, net	3,990.0
Regulatory Assets	437.0
Goodwill	1,579.0
Other Noncurrent Assets	134.0
Total Noncurrent Assets	6,140.0
Accounts Receivable Forgiven, net							18.0
Total Assets	6,770.0
Business Combination, Regulatory Assets Expensed					20.0			$ 20.0
Long-Term Debt Due Within One Year	14.0
Acquisition commitment write off expenses									101.0
Accounts Receivable Forgiven, AR Reserve				3.0					26.0
Accounts Payable	181.0
Less: fair value of assumed long-term debt outstanding, net	1,496.0
Taxes Accrued	44.0
Regulatory Liabilities	237.0
Other Current Liabilities	198.0
Regulatory Liabilities	628.0
Other Deferred Credits and Noncurrent Liabilities	150.0
Noncurrent Liabilities	2,274.0
Total cash consideration		$ 3,822.0
Cash and cash equivalents	142.0
Other Current Assets	65.0
Total Current Liabilities	674.0
Accounts Receivable Forgiven, Gross							44.0
Operating Revenues from external customers									512.0
Net income (loss)									(55.0)
Public Utilities, Customer Bill Credit Agreement, Net of Tax	40.0
Rhode Island Energy | RIPUC regulatory liability
Business Acquisition [Line Items]
Regulatory Liability				50.0			$ 50.0		50.0		$ 50.0
Rhode Island Energy | Accounts Receivable and Regulatory Asset write off
Business Acquisition [Line Items]
Acquisition commitment write off expenses									46.0
Rhode Island Energy | Accrued expenditures for the RIAG and decarbonation goals
Business Acquisition [Line Items]
Acquisition commitment write off expenses									5.0
Rhode Island Energy | Rhode Island Energy TSA
Business Acquisition [Line Items]
Other Nonrecurring (Income) Expense				$ 41.0					142.0
Rhode Island Energy | Accrual for RIAG Evaluation Fund
Business Acquisition [Line Items]
Other Nonrecurring (Income) Expense					$ 2.5
Rhode Island Energy | Accrual for TSA Markup
Business Acquisition [Line Items]
Other Nonrecurring (Income) Expense									$ 2.0
Rhode Island Energy | Electric Customer
Business Acquisition [Line Items]
Public Utilities, Customer Bill Credit Agreement, Total	33.0
Rhode Island Energy | Natural Gas Customer
Business Acquisition [Line Items]
Public Utilities, Customer Bill Credit Agreement, Total	$ 17.0
Forecast [Member]
Business Acquisition [Line Items]
Business Combination, Integration Related Costs												$ 408.0