Summary of Significant Accounting Policies and Other Matters - Investments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) mi	Dec. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 31, 2016
Schedule of Equity Method Investments [Line Items]
Supplemental retirement income plan assets	$ 465		$ 397
Deferred income plan assets	92		81
Other	3		2
Total investments	$ 1,816		2,065
Stagecoach
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%
Mountain Valley Pipeline
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 530		530
Mountain Valley Pipeline | Minimum
Schedule of Equity Method Investments [Line Items]
Aggregate project cost	5,800
Mountain Valley Pipeline | Maximum
Schedule of Equity Method Investments [Line Items]
Aggregate project cost	$ 6,000
Mountain Valley Pipeline | Mountain Valley Pipeline
Schedule of Equity Method Investments [Line Items]
Expected ownership percentage	8.80%
CET | Stagecoach
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 845		924
CET | Mountain Valley Pipeline
Schedule of Equity Method Investments [Line Items]
Equity method investments	342		602
CET | New York Transco, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	69		59
CECONY
Schedule of Equity Method Investments [Line Items]
Supplemental retirement income plan assets	439		371
Deferred income plan assets	92		81
Other	10		9
Total investments	541		461
CECONY | CET | Stagecoach
Schedule of Equity Method Investments [Line Items]
Equity method investments	0		0
CECONY | CET | Mountain Valley Pipeline
Schedule of Equity Method Investments [Line Items]
Equity method investments	0		0
CECONY | CET | New York Transco, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	0		0
CET Gas | Mountain Valley Pipeline
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 530		$ 530
Equity method investment, ownership percentage	11.30%
Carrying value of investment	$ 342	$ 662
Number of miles in pipeline construction | mi	77
Long -term firm transportation contract period	20 years
Cash flows discount rate	8.30%
Impairment loss	$ 320
Expected reduction in equity interest	8.80%
Impairment loss, after tax	$ 223
CET Gas | Mountain Valley Pipeline | Mountain Valley Pipeline
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage				12.50%