Other Material Contingencies - Total Guarantees (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 2,856,000,000	$ 2,547,000,000
NY Transco
Guarantor Obligations [Line Items]
Total guarantees, by type and term	1,358,000,000
Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	924,000,000
Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	499,000,000
Other
Guarantor Obligations [Line Items]
Total guarantees, by type and term	75,000,000
0 – 3 years
Guarantor Obligations [Line Items]
Total guarantees, by type and term	2,129,000,000
0 – 3 years | NY Transco
Guarantor Obligations [Line Items]
Total guarantees, by type and term	791,000,000
0 – 3 years | Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	792,000,000
0 – 3 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	471,000,000
0 – 3 years | Other
Guarantor Obligations [Line Items]
Total guarantees, by type and term	75,000,000
4 – 10 years
Guarantor Obligations [Line Items]
Total guarantees, by type and term	603,000,000
4 – 10 years | NY Transco
Guarantor Obligations [Line Items]
Total guarantees, by type and term	567,000,000
4 – 10 years | Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	35,000,000
4 – 10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	1,000,000
Greater than 10 years
Guarantor Obligations [Line Items]
Total guarantees, by type and term	124,000,000
Greater than 10 years | Energy transactions
Guarantor Obligations [Line Items]
Total guarantees, by type and term	97,000,000
Greater than 10 years | Renewable electric production projects
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 27,000,000