Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Steam) (Details) - CECONY - Steam - USD ($)	12 Months Ended				36 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory (assets) and liabilities					$ 37,000,000
Regulatory liabilities, amortization period					3 years
Negative revenue adjustments	$ 0	$ 0	$ 0	$ 0
Cost reconciliation, deferred net regulatory liabilities	14,000,000	8,000,000		42,000,000	$ 8,000,000
Cost reconciliations			17,000,000
Net utility plant reconciliations	0	$ 0	0	$ 100,000
Authorized return on common equity (percent)					9.30%
Earnings sharing (percentage)		9.90%			9.90%
Earnings sharing, threshold limit	$ 8,500,000	$ 7,800,000	$ 11,500,000
Earnings sharing, positive adjustment		4,000,000
Common equity ratio (percent)					48.00%
Other regulatory liabilities		$ 8,000,000			$ 8,000,000
Difference in property taxes (percent)					90.00%
Deferral, annual maximum (not more than) (percent)					0.10%
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes					$ (22,400,000)
Average rate base					$ 1,511,000,000
Weighted average cost of capital (after-tax) (percent)					7.10%
Actual return on common equity (percent)					9.82%
Cost of long-term debt (percent)		5.17%			5.17%
Year 1 | Production
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations					$ 1,752,000,000
Year 1 | Distribution
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations					6,000,000
Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)					1,000,000
Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes					19,800,000
Average rate base					$ 1,547,000,000
Weighted average cost of capital (after-tax) (percent)					7.13%
Actual return on common equity (percent)					10.88%
Cost of long-term debt (percent)		5.23%			5.23%
Year 2 | Production
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations					$ 1,732,000,000
Year 2 | Distribution
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations					11,000,000
Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)					1,000,000
Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes					20,300,000
Average rate base					$ 1,604,000,000
Weighted average cost of capital (after-tax) (percent)					7.21%
Actual return on common equity (percent)					10.54%
Cost of long-term debt (percent)		5.39%			5.39%
Year 3 | Production
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations					$ 1,720,000,000
Year 3 | Distribution
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations					25,000,000
Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)					1,000,000
Year 4
Public Utilities, General Disclosures [Line Items]
Base rate changes					$ 0
Actual return on common equity (percent)					9.51%