Other Material Contingencies - Total Guarantees (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 2,559,000,000	$ 2,547,000,000
NY Transco [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	1,360,000,000
Energy Transactions [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	958,000,000
Renewable Electric Production Projects [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	211,000,000
Other [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	30,000,000
0 - 3 years [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	2,424,000,000
0 - 3 years [Member] | NY Transco [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	1,360,000,000
0 - 3 years [Member] | Energy Transactions [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	830,000,000
0 - 3 years [Member] | Renewable Electric Production Projects [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	204,000,000
0 - 3 years [Member] | Other [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	30,000,000
4 - 10 years [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	38,000,000
4 - 10 years [Member] | Energy Transactions [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	38,000,000
Greater than 10 years [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	97,000,000
Greater than 10 years [Member] | Energy Transactions [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	90,000,000
Greater than 10 years [Member] | Renewable Electric Production Projects [Member]
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 7,000,000