Regulatory Matters - Regulatory Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory assets
Regulatory assets – noncurrent	$ 8,096	$ 9,142
Regulatory assets – current	132	138
Total Regulatory Assets	8,228	9,280
Regulatory liabilities
Regulatory liabilities – noncurrent	1,977	1,993
Regulatory liabilities—current	115	163
Total Regulatory Liabilities	2,092	2,156
Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	676	598
Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	128	155
Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	109	138
Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	99	105
Earnings sharing - electric and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	80	19
Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	76	46
New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	75	62
Variable-rate tax-exempt debt - cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	70	78
Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	49	58
Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	44	87
Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	32	21
Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	28	0
World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	21	41
Other
Regulatory liabilities
Regulatory liabilities – noncurrent	187	290
Refundable energy costs
Regulatory liabilities
Regulatory liabilities—current	64	128
Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities—current	45	30
Deferred derivative gains
Regulatory liabilities
Regulatory liabilities—current	6	5
Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	3,876	4,846
Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,350	2,259
Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	904	925
Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	253	219
Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	185	319
Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	50	57
Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	50	25
O&R property tax reconciliation
Regulatory assets
Regulatory assets – noncurrent	46	36
Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	45	66
Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	44	99
Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	44	63
Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	26	27
O&R transition bond charges
Regulatory assets
Regulatory assets – noncurrent	21	27
Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	16	19
Regulatory assets – current	19	41
Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	11	8
Other
Regulatory assets
Regulatory assets – noncurrent	175	147
Deferred derivative losses
Regulatory assets
Regulatory assets – current	113	97
Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	303	295
CECONY
Regulatory assets
Regulatory assets – noncurrent	7,482	8,457
Regulatory assets – current	121	132
Total Regulatory Assets	7,603	8,589
Regulatory liabilities
Regulatory liabilities – noncurrent	1,789	1,837
Regulatory liabilities—current	84	118
Total Regulatory Liabilities	1,873	1,955
CECONY | Allowance for cost of removal less salvage
Regulatory liabilities
Regulatory liabilities – noncurrent	570	499
CECONY | Base rate change deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	128	155
CECONY | Net unbilled revenue deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	109	138
CECONY | Prudence proceeding
Regulatory liabilities
Regulatory liabilities – noncurrent	99	105
CECONY | Earnings sharing - electric and steam
Regulatory liabilities
Regulatory liabilities – noncurrent	80	18
CECONY | Pension and other postretirement benefit deferrals
Regulatory liabilities
Regulatory liabilities – noncurrent	46	37
CECONY | New York State income tax rate change
Regulatory liabilities
Regulatory liabilities – noncurrent	72	59
CECONY | Variable-rate tax-exempt debt - cost rate reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	60	78
CECONY | Carrying charges on repair allowance and bonus depreciation
Regulatory liabilities
Regulatory liabilities – noncurrent	48	57
CECONY | Property tax refunds
Regulatory liabilities
Regulatory liabilities – noncurrent	44	87
CECONY | Net utility plant reconciliations
Regulatory liabilities
Regulatory liabilities – noncurrent	31	20
CECONY | Unrecognized other postretirement costs
Regulatory liabilities
Regulatory liabilities – noncurrent	28	0
CECONY | World Trade Center settlement proceeds
Regulatory liabilities
Regulatory liabilities – noncurrent	21	41
CECONY | Other
Regulatory liabilities
Regulatory liabilities – noncurrent	150	248
CECONY | Refundable energy costs
Regulatory liabilities
Regulatory liabilities—current	33	84
CECONY | Revenue decoupling mechanism
Regulatory liabilities
Regulatory liabilities—current	45	30
CECONY | Deferred derivative gains
Regulatory liabilities
Regulatory liabilities—current	6	4
CECONY | Unrecognized pension and other postretirement costs
Regulatory assets
Regulatory assets – noncurrent	3,697	4,609
CECONY | Future income tax
Regulatory assets
Regulatory assets – noncurrent	2,232	2,142
CECONY | Environmental remediation costs
Regulatory assets
Regulatory assets – noncurrent	800	820
CECONY | Revenue taxes
Regulatory assets
Regulatory assets – noncurrent	240	208
CECONY | Deferred storm costs
Regulatory assets
Regulatory assets – noncurrent	110	224
CECONY | Unamortized loss on reacquired debt
Regulatory assets
Regulatory assets – noncurrent	48	55
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – noncurrent	46	23
CECONY | Pension and other postretirement benefits deferrals
Regulatory assets
Regulatory assets – noncurrent	16	42
CECONY | Surcharge for New York State assessment
Regulatory assets
Regulatory assets – noncurrent	40	92
CECONY | Net electric deferrals
Regulatory assets
Regulatory assets – noncurrent	44	63
CECONY | Preferred stock redemption
Regulatory assets
Regulatory assets – noncurrent	26	27
CECONY | Recoverable energy costs
Regulatory assets
Regulatory assets – noncurrent	15	17
Regulatory assets – current	18	40
CECONY | Workers’ compensation
Regulatory assets
Regulatory assets – noncurrent	11	8
CECONY | Other
Regulatory assets
Regulatory assets – noncurrent	157	127
CECONY | Deferred derivative losses
Regulatory assets
Regulatory assets – current	103	92
CECONY | Property tax reconciliation
Regulatory liabilities
Regulatory liabilities – noncurrent	$ 303	$ 295