Regulatory Assets and Liabilities - Current and Non-Current Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 200	$ 192
Regulatory liabilities including deferred income taxes, noncurrent	1,755	1,753
Deferred income taxes regulatory	574	565
Regulatory liabilities including deferred income taxes, noncurrent	2,329	2,318
Support Services [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	1	3
Non By Passable Charge [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	9	22
Energy Efficiency Services [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	47	45
Gas Supply Charge And Deferred Natural Gas Cost [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14	6
Transmission Revenue Reconciliation Mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	9	5
Pension And Other Post-Retirement Benefits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	1	3
Regulatory liabilities including deferred income taxes, noncurrent	72	76
Other Post-Retirement Benefits Cost Deferrals [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14	14
Carrying Costs On Deferred Income Tax Bonus Depreciation [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15	15
Regulatory liabilities including deferred income taxes, noncurrent	91	95
Yankee DOE Refund [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	22	24
Merger Related Rate Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	2	3
Regulatory liabilities including deferred income taxes, noncurrent	20	21
Carrying Costs On Deferred Income Tax [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5	5
Regulatory liabilities including deferred income taxes, noncurrent	24	25
Revenue Decoupling Mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	3	9
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	58	38
Regulatory liabilities including deferred income taxes, noncurrent	66	69
Accrued Removal Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	1,126	1,117
Asset Sale Gain Account [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	9	9
Economic Development [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	34	35
Merger Capital Expense [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	14	15
Positive Benefit Adjustment [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	41	42
New York State Tax Rate Change [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	8	9
Post Term Amortization [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	3	3
Theoretical Reserve Flow Thru Impact [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	23	24
Deferred Property Tax [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	18	19
Net Plant Reconciliation [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	10	10
Variable Rate Debt [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	28	28
Rate Refund [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	26	26
Accumulated Deferred Investment Tax Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	14	15
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	11	13
Transmission Congestion Contracts [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	18	18
Middletown/Norwalk Local Transmission Network Service Collections [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	19	19
Low Income Programs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	48	46
Earning sharing provisions [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	22	12
Non-Firm Margin Sharing Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities including deferred income taxes, noncurrent	$ 10	$ 7