Regulatory Matters - Regulatory Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Regulatory assets
Regulatory assets – noncurrent	$ 8,445	$ 9,156
Regulatory assets – current	67	148
Total Regulatory Assets	8,512	9,304
Regulatory liabilities
Regulatory liabilities – noncurrent	1,940	1,993
Regulatory liabilities – current	165	187
Total Regulatory Liabilities	2,105	2,180
Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	633	598
Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	299	295
Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	134	155
Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	134	138
Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	100	105
Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	76	46
Variable-rate tax-exempt debt – cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	75	78
New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	67	62
Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	55	87
Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	50	58
Earnings sharing – electric and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	37	19
Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	31	21
World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	26	41
Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	20	0
Other
Regulatory liabilities
Regulatory liabilities – noncurrent	203	290
Refundable energy costs
Regulatory liabilities
Regulatory liabilities – current	99	128
Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities – current	41	30
Future income tax
Regulatory liabilities
Regulatory liabilities – current	20	24
Deferred derivative gains
Regulatory liabilities
Regulatory liabilities – current	5	5
Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	4,208	4,846
Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,363	2,273
Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	235	219
Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	218	319
Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	68	99
Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	52	57
Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	49	66
Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	49	63
O&R property tax reconciliation
Regulatory assets
Regulatory assets – noncurrent	45	36
Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	32	25
Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	26	27
O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	22	27
Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	4	19
Regulatory assets – current	1	41
Other
Regulatory assets
Regulatory assets – noncurrent	179	147
Deferred derivative losses
Regulatory assets
Regulatory assets – current	57	97
Future income tax
Regulatory assets
Regulatory assets – current	9	10
CECONY
Regulatory assets
Regulatory assets – noncurrent	7,820	8,481
Regulatory assets – current	53	132
Total Regulatory Assets	7,873	8,613
Regulatory liabilities
Regulatory liabilities – noncurrent	1,755	1,837
Regulatory liabilities – current	135	142
Total Regulatory Liabilities	1,890	1,979
CECONY | Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	530	499
CECONY | Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	299	295
CECONY | Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	134	155
CECONY | Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	134	138
CECONY | Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	100	105
CECONY | Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	46	37
CECONY | Variable-rate tax-exempt debt – cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	64	78
CECONY | New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	63	59
CECONY | Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	55	87
CECONY | Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	49	57
CECONY | Earnings sharing – electric and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	37	18
CECONY | Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	31	20
CECONY | World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	26	41
CECONY | Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	20	0
CECONY | Other
Regulatory liabilities
Regulatory liabilities – noncurrent	167	248
CECONY | Refundable energy costs
Regulatory liabilities
Regulatory liabilities – current	71	84
CECONY | Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities – current	39	30
CECONY | Future income tax
Regulatory liabilities
Regulatory liabilities – current	20	24
CECONY | Deferred derivative gains
Regulatory liabilities
Regulatory liabilities – current	5	4
CECONY | Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	4,008	4,609
CECONY | Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,254	2,166
CECONY | Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	223	208
CECONY | Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	137	224
CECONY | Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	63	92
CECONY | Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	50	55
CECONY | Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	21	42
CECONY | Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	49	63
CECONY | O&R property tax reconciliation
Regulatory assets
Regulatory assets – noncurrent	0	0
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	28	23
CECONY | Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	26	27
CECONY | O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	0	0
CECONY | Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	4	17
Regulatory assets – current	1	40
CECONY | Other
Regulatory assets
Regulatory assets – noncurrent	162	127
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – current	52	92
CECONY | Future income tax
Regulatory assets
Regulatory assets – current	0	0
Superfund Sites
Regulatory assets
Total Regulatory Assets	884	925
Superfund Sites | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	884	925
Superfund Sites | CECONY
Regulatory assets
Total Regulatory Assets	784	820
Superfund Sites | CECONY | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	784	820
Workers’ compensation
Regulatory assets
Total Regulatory Assets	11	8
Workers’ compensation | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	11	8
Workers’ compensation | CECONY
Regulatory assets
Total Regulatory Assets	11	8
Workers’ compensation | CECONY | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	$ 11	$ 8