Other Material Contingencies - Total Guarantees (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 2,576	$ 2,547
NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	1,359
Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	885
Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	302
Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	30
0-3 years
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	1,912
0-3 years | NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	946
0-3 years | Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	760
0-3 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	176
0-3 years | Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	30
4-10 years
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	531
4-10 years | NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	413
4-10 years | Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	36
4-10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	82
4-10 years | Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	0
Greater than 10 years
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	133
Greater than 10 years | NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	0
Greater than 10 years | Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	89
Greater than 10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	44
Greater than 10 years | Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 0