Derivative Instruments and Hedging Activities - Realized and Unrealized Gains (Losses) on Commodity Derivatives (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 166	$ (8)	$ 182	$ (6)
Total deferred gains/(losses)	0	(22)	27	(176)
Net deferred gains/(losses)	166	(30)	209	(182)
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	(24)	(1)	41	(92)
Gas purchased for resale
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	1	0	2	(3)
Non-utility revenue
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	(2)	1	1	5
Other operations and maintenance expense
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	2	2	4	(5)
Other interest expense
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	(25)	(4)	34	(89)
Deferred Derivative Gains, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	128	(2)	147	(3)
Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	38	(6)	35	(3)
Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	24	36	29	22
Recoverable Energy Costs, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(47)	(45)	(47)	(140)
Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	23	(13)	45	(58)
CECONY | Subsidiaries
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	153	(8)	168	(6)
Total deferred gains/(losses)	(1)	(23)	24	(164)
Net deferred gains/(losses)	152	(31)	192	(170)
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	2	2	4	(5)
CECONY | Gas purchased for resale | Subsidiaries
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	0	0	0	0
CECONY | Non-utility revenue | Subsidiaries
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	0	0	0	0
CECONY | Other operations and maintenance expense | Subsidiaries
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	2	2	4	(5)
CECONY | Other interest expense | Subsidiaries
Pre-tax gains/(losses) recognized in income
Total pre-tax gains/(losses) recognized in income	0	0	0	0
CECONY | Deferred Derivative Gains, Current | Subsidiaries
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	119	(2)	136	(3)
CECONY | Deferred Derivative Gains, Noncurrent | Subsidiaries
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	34	(6)	32	(3)
CECONY | Deferred Derivative Losses, Current | Subsidiaries
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	22	32	25	20
CECONY | Recoverable Energy Costs, Current | Subsidiaries
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(45)	(40)	(43)	(126)
CECONY | Deferred Derivative Losses, Noncurrent | Subsidiaries
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 22	$ (15)	$ 42	$ (58)