Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Steam) (Detail) (USD $)	12 Months Ended						36 Months Ended	12 Months Ended		36 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Cost reconciliation, deferred net regulatory liabilities	$ 910,000,000	$ 871,000,000
CECONY [Member]
Public Utilities, General Disclosures [Line Items]
Cost reconciliation, deferred net regulatory liabilities	863,000,000	825,000,000
CECONY [Member] | Steam [Member]
Public Utilities, General Disclosures [Line Items]
Base rate changes	(22,400,000)		17,800,000		49,500,000	49,500,000
Base rate change through surcharge			31,700,000
Amortizations to income of net regulatory (assets) and liabilities							(20,100,000)
Negative revenue adjustments	0	0		0
Cost reconciliation, deferred net regulatory liabilities	42,000,000	17,000,000		12,000,000
Deferred regulatory liability	1,100,000	0		200,000
Average rate base	1,511,000,000		1,613,000,000		1,603,000,000	1,589,000,000
Authorized return on common equity, austerity measures		2,000,000		3,000,000
Earnings sharing, threshold limit	0	500,000
Weighted average cost of capital (after-tax)	7.10%		7.46%		7.46%	7.46%
Cost of long-term debt	5.17%		5.57%		5.57%	5.57%	5.57%
Authorized return on common equity	9.30%		9.60%		9.60%	9.60%
Common equity ratio	48.00%		48.00%		48.00%	48.00%
Earnings sharing percentage	9.90%		10.15%		10.15%	10.35%	10.15%
CECONY [Member] | Steam [Member] | Production [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability	1,752,000,000		433,000,000		426,000,000	415,000,000
CECONY [Member] | Steam [Member] | Distribution [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability	6,000,000		543,000,000		534,000,000	521,000,000
CECONY [Member] | Steam [Member] | Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Base rate changes								20,300,000	19,800,000
Amortizations to income of net regulatory (assets) and liabilities										37,000,000
Average rate base								1,604,000,000	1,547,000,000
Weighted average cost of capital (after-tax)								7.21%	7.13%
Cost of long-term debt								5.39%	5.23%	5.39%
Authorized return on common equity								9.30%	9.30%
Common equity ratio								48.00%	48.00%
Earnings sharing percentage								9.90%	9.90%	9.90%
CECONY [Member] | Steam [Member] | Scenario, Forecast [Member] | Production [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability								1,720,000,000	1,732,000,000
CECONY [Member] | Steam [Member] | Scenario, Forecast [Member] | Distribution [Member]
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability								25,000,000	11,000,000
CECONY [Member] | Maximum [Member] | Steam [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties	1,000,000		1,000,000		1,000,000	1,000,000
CECONY [Member] | Maximum [Member] | Steam [Member] | Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties								$ 1,000,000	$ 1,000,000