Regulatory Matters - Regulatory Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Regulatory assets
Regulatory assets – noncurrent	$ 7,544	$ 8,096
Regulatory assets – current	119	132
Total Regulatory Assets	7,663	8,228
Regulatory liabilities
Regulatory liabilities – noncurrent	1,920	1,977
Regulatory liabilities – current	123	115
Total Regulatory Liabilities	2,043	2,092
Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	713	676
Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	205	303
Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	163	76
Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	121	109
Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	96	99
Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	91	28
New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	66	75
Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	62	128
Variable-rate tax-exempt debt – cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	60	70
Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	57	49
Earnings sharing - electric, gas and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	34	80
Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	27	32
Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	12	44
World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	5	21
Other
Regulatory liabilities
Regulatory liabilities – noncurrent	208	187
Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities – current	74	45
Refundable energy costs
Regulatory liabilities
Regulatory liabilities – current	37	64
Deferred derivative gains
Regulatory liabilities
Regulatory liabilities – current	12	6
Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	3,369	3,876
Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,429	2,350
Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	298	253
Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	89	185
Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	55	50
Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	45	50
Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	43	44
O&R property tax reconciliation
Regulatory assets
Regulatory assets – noncurrent	39	46
Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	34	45
Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	29	44
Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	25	26
O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	16	21
Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	7	16
Regulatory assets – current	25	19
Other
Regulatory assets
Regulatory assets – noncurrent	228	175
Deferred derivative losses
Regulatory assets
Regulatory assets – current	94	113
CECONY
Regulatory assets
Regulatory assets – noncurrent	6,986	7,482
Regulatory assets – current	111	121
Total Regulatory Assets	7,097	7,603
Regulatory liabilities
Regulatory liabilities – noncurrent	1,725	1,789
Regulatory liabilities – current	96	84
Total Regulatory Liabilities	1,821	1,873
CECONY | Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	602	570
CECONY | Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	205	303
CECONY | Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	130	46
CECONY | Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	121	109
CECONY | Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	96	99
CECONY | Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	91	28
CECONY | New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	64	72
CECONY | Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	62	128
CECONY | Variable-rate tax-exempt debt – cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	52	60
CECONY | Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	56	48
CECONY | Earnings sharing - electric, gas and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	26	80
CECONY | Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	27	31
CECONY | Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	12	44
CECONY | World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	5	21
CECONY | Other
Regulatory liabilities
Regulatory liabilities – noncurrent	176	150
CECONY | Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities – current	70	45
CECONY | Refundable energy costs
Regulatory liabilities
Regulatory liabilities – current	18	33
CECONY | Deferred derivative gains
Regulatory liabilities
Regulatory liabilities – current	8	6
CECONY | Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	3,220	3,697
CECONY | Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,312	2,232
CECONY | Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	283	240
CECONY | Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	30	110
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	49	46
CECONY | Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	43	48
CECONY | Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	40	40
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	3	16
CECONY | Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	29	44
CECONY | Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	25	26
CECONY | O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	0	0
CECONY | Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	5	15
Regulatory assets – current	24	18
CECONY | Other
Regulatory assets
Regulatory assets – noncurrent	212	157
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – current	87	103
Superfund Sites
Regulatory assets
Total Regulatory Assets	823	904
Superfund Sites | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	823	904
Superfund Sites | CECONY
Regulatory assets
Total Regulatory Assets	720	800
Superfund Sites | CECONY | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	720	800
Workers’ compensation
Regulatory assets
Total Regulatory Assets	15	11
Workers’ compensation | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	15	11
Workers’ compensation | CECONY
Regulatory assets
Total Regulatory Assets	15	11
Workers’ compensation | CECONY | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	$ 15	$ 11