Other Material Contingencies - Guarantees (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 2,128	$ 2,073
Con Edison Transmission
Guarantor Obligations [Line Items]
Total guarantees, by type and term	1,146
Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	663
Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	196
Other
Guarantor Obligations [Line Items]
Total guarantees, by type and term	123
0 – 3 years
Guarantor Obligations [Line Items]
Total guarantees, by type and term	1,463
0 – 3 years | Con Edison Transmission
Guarantor Obligations [Line Items]
Total guarantees, by type and term	742
0 – 3 years | Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	426
0 – 3 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	172
0 – 3 years | Other
Guarantor Obligations [Line Items]
Total guarantees, by type and term	123
4 – 10 years
Guarantor Obligations [Line Items]
Total guarantees, by type and term	429
4 – 10 years | Con Edison Transmission
Guarantor Obligations [Line Items]
Total guarantees, by type and term	404
4 – 10 years | Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	25
4 – 10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	0
4 – 10 years | Other
Guarantor Obligations [Line Items]
Total guarantees, by type and term	0
Greater than 10 years
Guarantor Obligations [Line Items]
Total guarantees, by type and term	236
Greater than 10 years | Con Edison Transmission
Guarantor Obligations [Line Items]
Total guarantees, by type and term	0
Greater than 10 years | Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	212
Greater than 10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	24
Greater than 10 years | Other
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 0