Variable Interest Entities - Additional Information (Detail)		1 Months Ended	9 Months Ended
	Sep. 30, 2015 USD ($) MW	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) MW	Dec. 31, 2014 USD ($) entity
Variable Interest Entity [Line Items]
Investments	$ 874,000,000	$ 874,000,000	$ 874,000,000	$ 816,000,000
Cogen Technologies Linden Venture, LP
Variable Interest Entity [Line Items]
Maximum exposure to loss	255,000,000	255,000,000	255,000,000
Brooklyn Navy Yard Cogeneration Partners, LP
Variable Interest Entity [Line Items]
Maximum exposure to loss	83,000,000	83,000,000	83,000,000
Indeck Energy Services of Corinth, Inc
Variable Interest Entity [Line Items]
Maximum exposure to loss	25,000,000	25,000,000	$ 25,000,000
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Percentage of variable interests			50.00%
CECONY
Variable Interest Entity [Line Items]
Number of potential VIEs, long-term electricity purchase agreements | entity				3
Investments	281,000,000	$ 281,000,000	$ 281,000,000	$ 271,000,000
Panoche Valley | Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Percentage of variable interests		50.00%
Investments	33,000,000	$ 33,000,000	33,000,000
Notes receivable	25,000,000	25,000,000	25,000,000
California | Panoche Valley | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Maximum exposure to loss	$ 58,000,000	$ 58,000,000	$ 58,000,000
Generating Capacity (MWs AC) | MW	247		124