Derivative Instruments and Hedging Activities - Realized and Unrealized Gains or Losses on Commodity Derivatives (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 3	$ 24
Total deferred gains/(losses)	(99)	(188)
Net deferred gains/(losses)	(96)	(164)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	8	(203)
Purchased power expense
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	(101)
Unrealized gain/(loss) on derivatives		11
Gas purchased for resale
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	3	(112)
Non-utility revenue
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	5	9
Other operations and maintenance expense
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	1
Unrealized gain/(loss) on derivatives		1
Non-utility operating revenue
Pre-tax gain/(loss) recognized in income
Unrealized gain/(loss) on derivatives		(5)
Deferred Derivative Gains,Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	3	23
Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	0	1
Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	51	22
Recoverable Energy Costs, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(154)	(212)
Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	4	2
CECONY
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	4	20
Total deferred gains/(losses)	(90)	(172)
Net deferred gains/(losses)	(86)	(152)
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	1
CECONY | Purchased power expense
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	0
CECONY | Gas purchased for resale
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	0
CECONY | Non-utility revenue
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	0
CECONY | Other operations and maintenance expense
Pre-tax gain/(loss) recognized in income
Total pre-tax gain/(loss) recognized in income	0	1
Unrealized gain/(loss) on derivatives		1
CECONY | Deferred Derivative Gains,Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	4	18
CECONY | Deferred Derivative Gains, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	0	2
CECONY | Deferred Derivative Losses, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	49	18
CECONY | Recoverable Energy Costs, Current
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	(144)	(194)
CECONY | Deferred Derivative Losses, Noncurrent
Pre-tax gains/(losses) deferred in accordance with accounting rules for regulated operations:
Total deferred gains/(losses)	$ 5	$ 4