Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Steam) (Detail) - USD ($)	12 Months Ended				36 Months Ended	39 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2016	Dec. 31, 2013
Public Utilities, General Disclosures [Line Items]
Cost reconciliation, deferred net regulatory liabilities	$ 947,000,000	$ 910,000,000
Cost reconciliations, deferred net regulatory assets	836,000,000	615,000,000
CECONY
Public Utilities, General Disclosures [Line Items]
Cost reconciliation, deferred net regulatory liabilities	899,000,000	863,000,000
Cost reconciliations, deferred net regulatory assets	752,000,000	574,000,000
Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Negative revenue adjustments	0	0	$ 0	$ 0
Cost reconciliation, deferred net regulatory liabilities		42,000,000	17,000,000	12,000,000		$ 17,000,000
Cost reconciliations, deferred net regulatory assets	17,000,000
Deferred regulatory liability (reduction)		1,100,000	0	200,000
Authorized return on common equity, austerity measures			2,000,000	$ 3,000,000
Earnings sharing, threshold limit	17,100,000	$ 0	$ 500,000
Potential refund from customers	$ 6,000,000
Difference in property taxes (percent)	90.00%		80.00%
Deferral, annual maximum (not more than) (percent)	0.10%
Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Other regulatory liabilities					$ 8,000,000
Steam | CECONY | Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)						1,000,000
Steam | CECONY | Maximum | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)					1,000,000
Year 1 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Base rate changes						49,500,000
Amortization to income of net regulatory (assets) and liabilities						(20,100,000)
Average rate base						$ 1,589,000,000
Weighted average cost of capital (after-tax) (percent)						7.46%
Authorized return on common equity (percent)						9.60%
Earnings sharing (percentage)			10.35%			10.35%
Cost of long-term debt (percent)			5.57%			5.57%
Common equity ratio (percent)						48.00%
Year 1 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes					(22,400,000)
Amortization to income of net regulatory (assets) and liabilities					37,000,000
Average rate base					$ 1,511,000,000
Weighted average cost of capital (after-tax) (percent)					7.10%
Authorized return on common equity (percent)					9.30%
Earnings sharing (percentage)					9.90%
Cost of long-term debt (percent)					5.17%
Common equity ratio (percent)					48.00%
Year 2 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 49,500,000
Amortization to income of net regulatory (assets) and liabilities						(20,100,000)
Average rate base						$ 1,603,000,000
Weighted average cost of capital (after-tax) (percent)						7.46%
Authorized return on common equity (percent)						9.60%
Earnings sharing (percentage)			10.15%			10.15%
Cost of long-term debt (percent)			5.57%			5.57%
Common equity ratio (percent)						48.00%
Year 2 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes					$ 19,800,000
Amortization to income of net regulatory (assets) and liabilities					37,000,000
Average rate base					$ 1,547,000,000
Weighted average cost of capital (after-tax) (percent)					7.13%
Authorized return on common equity (percent)					9.30%
Earnings sharing (percentage)					9.90%
Cost of long-term debt (percent)					5.23%
Common equity ratio (percent)					48.00%
Year 3 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 17,800,000
Base rate change through surcharge						31,700,000
Amortization to income of net regulatory (assets) and liabilities						(20,100,000)
Average rate base						$ 1,613,000,000
Weighted average cost of capital (after-tax) (percent)						7.46%
Authorized return on common equity (percent)						9.60%
Earnings sharing (percentage)			10.15%			10.15%
Cost of long-term debt (percent)			5.57%			5.57%
Common equity ratio (percent)						48.00%
Year 3 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes					$ 20,300,000
Amortization to income of net regulatory (assets) and liabilities					37,000,000
Average rate base					$ 1,604,000,000
Weighted average cost of capital (after-tax) (percent)					7.21%
Authorized return on common equity (percent)					9.30%
Earnings sharing (percentage)					9.90%
Cost of long-term debt (percent)					5.39%
Common equity ratio (percent)					48.00%
Production | Year 1 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)						$ 415,000,000
Production | Year 1 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)					$ 1,752,000,000
Production | Year 2 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)						426,000,000
Production | Year 2 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)					1,732,000,000
Production | Year 3 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)						433,000,000
Production | Year 3 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)					1,720,000,000
Distribution | Year 1 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)						521,000,000
Distribution | Year 1 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)					6,000,000
Distribution | Year 2 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)						534,000,000
Distribution | Year 2 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)					11,000,000
Distribution | Year 3 | Steam | CECONY
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)						$ 543,000,000
Distribution | Year 3 | Steam | CECONY | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred regulatory liability (reduction)					$ 25,000,000