Environmental Liabilities - Additional Information (Detail) $ in Millions					1 Months Ended		6 Months Ended
	Jul. 05, 2017 USD ($)	Sep. 11, 2014 USD ($)	Aug. 14, 2013 USD ($)	Sep. 09, 2011 USD ($)	Nov. 30, 2014 USD ($)	Jul. 31, 2011 USD ($)	Jun. 30, 2018 USD ($) site Location	Dec. 31, 2017 USD ($)	Aug. 04, 2016 USD ($)
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations | site							25
Number of sites where gas was manufactured in the past | site							53
Number of sites for which we have entered into consent orders to investigate and remediate | site							49
Costs related to investigation and remediation							$ 385	$ 389
Accrual for environmental loss contingencies		$ 27					$ 22
Damages for incurred costs payment amount		$ 22
Refund of environmental remediation cost paid					$ 5
Number of sites with modified decision | site							9
First Energy [Member]
Environmental Exit Cost [Line Items]
Reasonably possible loss, in additional reserve, net of tax						$ 60
Environmental costs paid				$ 30
First Energy [Member] | Past Costs [Member]
Environmental Exit Cost [Line Items]
Accrual for environmental loss contingencies				27
First Energy [Member] | Pre-judgment Interest [Member]
Environmental Exit Cost [Line Items]
Environmental costs paid				$ 3
Century Indemnity and OneBeacon [Member]
Environmental Exit Cost [Line Items]
Reasonably possible loss, in additional reserve, net of tax			$ 89
Number of hazardous waste sites | Location							22
Legal discovery process expected closing year description							Century Indemnity and OneBeacon have answered, admitting issuance of the excess policies, but contesting coverage and providing documentation proving they received notice of the claims in the 1990s
Asnat [Member] | Environmental Remediation Activities [Member]
Environmental Exit Cost [Line Items]
Pretrial memorandum claiming damages	$ 10
United Illuminating Company (UI)
Environmental Exit Cost [Line Items]
Costs related to investigation and remediation							$ 23	25	$ 30
Maximum [Member] | Century Indemnity and OneBeacon [Member]
Environmental Exit Cost [Line Items]
Reasonably possible loss, in additional reserve, net of tax			$ 282
New York State Registry [Member]
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations | site							15
Number of sites where gas was manufactured in the past | site							8
Maine's Uncontrolled Sites Program [Member]
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations | site							6
Number of sites where gas was manufactured in the past | site							2
Massachusetts Non- Priority Confirmed Disposal Site List [Member]
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations | site							1
National Priorities List [Member]
Environmental Exit Cost [Line Items]
Number of sites with potential remediation obligations | site							9
Ten of Twenty-five Sites [Member]
Environmental Exit Cost [Line Items]
Estimated environmental liability							$ 5
Another Ten Sites [Member]
Environmental Exit Cost [Line Items]
Estimated environmental liability							8
Another Ten Sites [Member] | Minimum [Member]
Environmental Exit Cost [Line Items]
Estimated environmental liability							12
Another Ten Sites [Member] | Maximum [Member]
Environmental Exit Cost [Line Items]
Estimated environmental liability							$ 21
New York Voluntary Cleanup Program [Member]
Environmental Exit Cost [Line Items]
Number of sites where gas was manufactured in the past | site							12
Maine's Voluntary Response Action Program [Member]
Environmental Exit Cost [Line Items]
Number of sites where gas was manufactured in the past | site							3
Manufactured Gas Plants | Connecticut [Member]
Environmental Exit Cost [Line Items]
Costs related to investigation and remediation							$ 99	$ 100
Manufactured Gas Plants | Minimum [Member]
Environmental Exit Cost [Line Items]
Costs related to investigation and remediation							213
Manufactured Gas Plants | Maximum [Member]
Environmental Exit Cost [Line Items]
Costs related to investigation and remediation							$ 442