Environmental Liabilities (Details)	9 Months Ended	12 Months Ended		98 Months Ended
	Sep. 30, 2024 USD ($) site	Dec. 31, 2011 site	Dec. 31, 2008 site	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Aug. 04, 2016 USD ($)
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations	24
Number of additional sites with liability recorded	9
Number of sites where gas was manufactured in the past	53
Number of sites for which we have entered into consent orders to investigate and remediate	41
Accrual related to investigation and remediation | $	$ 249,000,000			$ 249,000,000	$ 250,000,000
Accrual related to investigation and remediation, amount recorded to date | $				35,000,000
FirstEnergy
Environmental Exit Cost [Line Items]
Number of sites in dispute		16
United Illuminating Company (UI)
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	$ 20,000,000			20,000,000	19,000,000
Cost of investigation and remediation | $						$ 30,000,000
New York State Registry
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations	16
Number of sites where gas was manufactured in the past	6
Maine's Uncontrolled Sites Program
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations	1
Number of sites where gas was manufactured in the past	0
Brownfield Cleanup Program
Environmental Exit Cost [Line Items]
Number of sites with liability recorded	7
Number of sites where gas was manufactured in the past	2
Massachusetts Non- Priority Confirmed Disposal Site List
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations	1
National Priorities List
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations	5
Ten of Twenty-five Sites
Environmental Exit Cost [Line Items]
Estimated environmental liability | $	$ 9,000,000			9,000,000
New York Voluntary Cleanup Program
Environmental Exit Cost [Line Items]
Number of sits note expected to incur additional liabilities	17
Number of sites where gas was manufactured in the past	39
Another Ten Sites
Environmental Exit Cost [Line Items]
Estimated environmental liability | $	$ 7,000,000			7,000,000
Another Ten Sites | Min.
Environmental Exit Cost [Line Items]
Estimated environmental liability | $	15,000,000			15,000,000
Another Ten Sites | Max.
Environmental Exit Cost [Line Items]
Estimated environmental liability | $	$ 23,000,000			23,000,000
Sites With Individual NYSDEC Orders Of Consent
Environmental Exit Cost [Line Items]
Number of sites where gas was manufactured in the past	1
Maine's Voluntary Response Action Program
Environmental Exit Cost [Line Items]
Number of sites where gas was manufactured in the past	2
Manufactured Gas Plants | Connecticut
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	$ 113,000,000			113,000,000	$ 112,000,000
Manufactured Gas Plants | Min.
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	114,000,000			114,000,000
Manufactured Gas Plants | Max.
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	213,000,000			213,000,000
Properties Where MGPs Had Historically Operated
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	0			0
MGP Sites | FirstEnergy
Environmental Exit Cost [Line Items]
Number of sites in dispute			2
NYSEG | FirstEnergy
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	8,000,000			8,000,000
RG&E | FirstEnergy
Environmental Exit Cost [Line Items]
Accrual related to investigation and remediation | $	$ 5,000,000			$ 5,000,000