Other Material Contingencies - Total Guarantees (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 2,547,000,000	$ 1,331,000,000
NY Transco [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	1,361,000,000
Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	901,000,000
Renewable Electric Production Projects [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	255,000,000
Other [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	30,000,000
0 - 3 years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	2,413,000,000
0 - 3 years [Member] | NY Transco [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	1,361,000,000
0 - 3 years [Member] | Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	774,000,000
0 - 3 years [Member] | Renewable Electric Production Projects [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	248,000,000
0 - 3 years [Member] | Other [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	30,000,000
4 - 10 years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	31,000,000
4 - 10 years [Member] | Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	31,000,000
Greater than 10 years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	103,000,000
Greater than 10 years [Member] | Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	96,000,000
Greater than 10 years [Member] | Renewable Electric Production Projects [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 7,000,000