Regulatory Assets and Liabilities - Current and Non-Current Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 3,556	$ 3,523
Less: current portion	214	242
Total non-current regulatory liabilities	3,342	3,281
Energy efficiency portfolio standard
Regulatory Liabilities [Line Items]
Total regulatory liabilities	72	72
Gas supply charge and deferred natural gas cost
Regulatory Liabilities [Line Items]
Total regulatory liabilities	4	11
Pension and other post-retirement benefits cost deferrals
Regulatory Liabilities [Line Items]
Total regulatory liabilities	69	80
Carrying costs on deferred income tax bonus depreciation
Regulatory Liabilities [Line Items]
Total regulatory liabilities	32	49
Carrying costs on deferred income tax - Mixed Services 263(a)
Regulatory Liabilities [Line Items]
Total regulatory liabilities	11	15
2017 Tax Act
Regulatory Liabilities [Line Items]
Total regulatory liabilities	1,550	1,548
Revenue decoupling mechanism
Regulatory Liabilities [Line Items]
Total regulatory liabilities	10	17
Accrued removal obligations
Regulatory Liabilities [Line Items]
Total regulatory liabilities	1,181	1,173
Asset sale gain account
Regulatory Liabilities [Line Items]
Total regulatory liabilities	10	10
Economic development
Regulatory Liabilities [Line Items]
Total regulatory liabilities	27	27
Positive benefit adjustment
Regulatory Liabilities [Line Items]
Total regulatory liabilities	35	37
Theoretical reserve flow thru impact
Regulatory Liabilities [Line Items]
Total regulatory liabilities	10	14
Deferred property tax
Regulatory Liabilities [Line Items]
Total regulatory liabilities	52	17
Net plant reconciliation
Regulatory Liabilities [Line Items]
Total regulatory liabilities	23	23
Debt rate reconciliation
Regulatory Liabilities [Line Items]
Total regulatory liabilities	83	67
Rate refund – FERC ROE proceeding
Regulatory Liabilities [Line Items]
Total regulatory liabilities	33	32
Transmission congestion contracts
Regulatory Liabilities [Line Items]
Total regulatory liabilities	24	23
Merger-related rate credits
Regulatory Liabilities [Line Items]
Total regulatory liabilities	14	16
Accumulated deferred investment tax credits
Regulatory Liabilities [Line Items]
Total regulatory liabilities	26	13
Asset retirement obligation
Regulatory Liabilities [Line Items]
Total regulatory liabilities	16	14
Earning sharing provisions
Regulatory Liabilities [Line Items]
Total regulatory liabilities	26	28
Middletown/Norwalk local transmission network service collections
Regulatory Liabilities [Line Items]
Total regulatory liabilities	18	18
Low income programs
Regulatory Liabilities [Line Items]
Total regulatory liabilities	29	33
Non-firm margin sharing credits
Regulatory Liabilities [Line Items]
Total regulatory liabilities	15	16
New York 2018 winter storm settlement
Regulatory Liabilities [Line Items]
Total regulatory liabilities	11	11
Other
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 175	$ 159