Regulatory Matters - Regulatory Assets and Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Regulatory assets
Regulatory assets – noncurrent	$ 7,930	$ 8,096
Regulatory assets – current	152	132
Total Regulatory Assets	8,082	8,228
Regulatory liabilities
Regulatory liabilities – noncurrent	1,858	1,977
Regulatory liabilities – current	125	115
Total Regulatory Liabilities	1,983	2,092
Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	693	676
Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	259	303
Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	98	99
Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	96	28
Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	95	128
Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	89	76
New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	71	75
Variable-rate tax-exempt debt – cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	67	70
Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	51	49
Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	44	109
Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	33	44
Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	30	32
Earnings sharing – electric and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	27	80
World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	15	21
Other
Regulatory liabilities
Regulatory liabilities – noncurrent	190	187
Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities – current	69	45
Refundable energy costs
Regulatory liabilities
Regulatory liabilities – current	53	64
Deferred derivative gains
Regulatory liabilities
Regulatory liabilities – current	3	6
Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	3,663	3,876
Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,363	2,350
Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	293	253
Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	154	185
Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	106	50
Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	55	44
Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	48	50
O&R property tax reconciliation
Regulatory assets
Regulatory assets – noncurrent	44	46
Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	41	45
Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	39	44
Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	26	26
O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	19	21
Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	0	16
Regulatory assets – current	9	19
Other
Regulatory assets
Regulatory assets – noncurrent	212	175
Deferred derivative losses
Regulatory assets
Regulatory assets – current	143	113
CECONY
Regulatory assets
Regulatory assets – noncurrent	7,336	7,482
Regulatory assets – current	139	121
Total Regulatory Assets	7,475	7,603
Regulatory liabilities
Regulatory liabilities – noncurrent	1,672	1,789
Regulatory liabilities – current	97	84
Total Regulatory Liabilities	1,769	1,873
CECONY | Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	586	570
CECONY | Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	259	303
CECONY | Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	98	99
CECONY | Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	96	28
CECONY | Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	95	128
CECONY | Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	59	46
CECONY | New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	69	72
CECONY | Variable-rate tax-exempt debt – cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	58	60
CECONY | Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	49	48
CECONY | Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	44	109
CECONY | Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	33	44
CECONY | Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	30	31
CECONY | Earnings sharing – electric and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	27	80
CECONY | World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	15	21
CECONY | Other
Regulatory liabilities
Regulatory liabilities – noncurrent	154	150
CECONY | Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities – current	69	45
CECONY | Refundable energy costs
Regulatory liabilities
Regulatory liabilities – current	26	33
CECONY | Deferred derivative gains
Regulatory liabilities
Regulatory liabilities – current	2	6
CECONY | Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	3,502	3,697
CECONY | Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,246	2,232
CECONY | Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	279	240
CECONY | Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	84	110
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	97	46
CECONY | Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	50	40
CECONY | Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	46	48
CECONY | O&R property tax reconciliation
Regulatory assets
Regulatory assets – noncurrent	0	0
CECONY | Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	11	16
CECONY | Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	39	44
CECONY | Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	26	26
CECONY | O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	0	0
CECONY | Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	0	15
Regulatory assets – current	8	18
CECONY | Other
Regulatory assets
Regulatory assets – noncurrent	194	157
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – current	131	103
Superfund Sites
Regulatory assets
Total Regulatory Assets	851	904
Superfund Sites | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	851	904
Superfund Sites | CECONY
Regulatory assets
Total Regulatory Assets	746	800
Superfund Sites | CECONY | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	746	800
Workers’ compensation
Regulatory assets
Total Regulatory Assets	16	11
Workers’ compensation | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	16	11
Workers’ compensation | CECONY
Regulatory assets
Total Regulatory Assets	16	11
Workers’ compensation | CECONY | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	$ 16	$ 11