Other Material Contingencies - Total Guarantees (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 2,529	$ 2,547
NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	1,359
Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	871
Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	269
Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	30
0-3 years
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	2,293
0-3 years | NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	1,359
0-3 years | Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	739
0-3 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	165
0-3 years | Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	30
4-10 years
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	92
4-10 years | NY Transco
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	0
4-10 years | Energy transactions
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	42
4-10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	50
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	144
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	90
Greater than 10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	54
Greater than 10 years | Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 0