Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Electric) (Detail) (USD $)	9 Months Ended	12 Months Ended						36 Months Ended	1 Months Ended	12 Months Ended	24 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2013	Jan. 31, 2016	Dec. 31, 2015	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Cost reconciliations, deferred net regulatory assets	$ 604,000,000	$ 615,000,000	$ 604,000,000
Cost reconciliations, deferred net regulatory liabilities	871,000,000	910,000,000	871,000,000
CECONY [Member]
Public Utilities, General Disclosures [Line Items]
Cost reconciliations, deferred net regulatory assets	569,000,000	574,000,000	569,000,000
Cost reconciliations, deferred net regulatory liabilities	825,000,000	863,000,000	825,000,000
CECONY [Member] | Electric Transmission [Member]
Public Utilities, General Disclosures [Line Items]
Deferred revenues	34,000,000		34,000,000		59,000,000
Cost reconciliations, deferred net regulatory assets	35,000,000		35,000,000
Base rate changes		(76,200,000)		287,000,000		420,000,000	420,000,000
Amortizations to income of net regulatory (assets) and liabilities								(75,300,000)
Deferred major storm costs		107,000,000
Retention of annual transmission congestion revenues	90,000,000	90,000,000		120,000,000		120,000,000	120,000,000
Negative revenue adjustments		5,000,000	0		0
Cost reconciliations, deferred net regulatory liabilities		57,000,000			146,000,000
Deferred regulatory liability			7,000,000		7,000,000
Average rate base		17,323,000,000		16,826,000,000		15,987,000,000	14,887,000,000
Weighted average cost of capital (after-tax)		7.05%		7.76%		7.76%	7.76%
Authorized return on common equity		9.20%		10.15%		10.15%	10.15%
Earnings sharing percentage		9.80%		10.65%		10.65%	11.15%	10.65%
Authorized return on common equity, austerity measures				13,000,000		20,000,000	27,000,000
Earnings sharing, threshold limit		0	17,500,000
Cost of long-term debt		5.17%		5.65%		5.65%	5.65%	5.65%
Common equity ratio		48.00%		48.00%		48.00%	48.00%
CECONY [Member] | Electric Transmission [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties		400,000,000		350,000,000		350,000,000	350,000,000
CECONY [Member] | Electric Transmission [Member] | T&D [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability		16,869,000,000		15,414,000,000		14,742,000,000	13,818,000,000
CECONY [Member] | Electric Transmission [Member] | ERP [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability				115,000,000		25,000,000
CECONY [Member] | Electric Transmission [Member] | Other [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability		2,034,000,000		1,650,000,000		1,565,000,000	1,487,000,000
CECONY [Member] | Electric Transmission [Member] | Storm Hardening [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability		89,000,000
CECONY [Member] | Electric Transmission [Member] | Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Base rate changes										124,000,000
Amortizations to income of net regulatory (assets) and liabilities											(37,000,000)
Deferred major storm costs										107,000,000	107,000,000
Retention of annual transmission congestion revenues										90,000,000
Average rate base										18,113,000,000
Weighted average cost of capital (after-tax)										7.08%
Authorized return on common equity									10.00%	9.20%
Earnings sharing percentage										9.80%	9.80%
Cost of long-term debt										5.23%	5.23%
Common equity ratio									48.00%	48.00%
CECONY [Member] | Electric Transmission [Member] | Scenario, Forecast [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties										400,000,000
CECONY [Member] | Electric Transmission [Member] | Scenario, Forecast [Member] | T&D [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability										17,401,000,000
CECONY [Member] | Electric Transmission [Member] | Scenario, Forecast [Member] | Other [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability										2,102,000,000
CECONY [Member] | Electric Transmission [Member] | Scenario, Forecast [Member] | Storm Hardening [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability										$ 177,000,000