Other Material Contingencies - Total Guarantees (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 823	$ 760
Financial And Performance Guarantee For Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	764
Financial And Performance Guarantee For Intra-Company [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	16
Financial And Performance Guarantee Other [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	43
Guarantee Duration, 0 - 3 Years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	749
Guarantee Duration, 0 - 3 Years [Member] | Financial And Performance Guarantee For Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	696
Guarantee Duration, 0 - 3 Years [Member] | Financial And Performance Guarantee For Intra-Company [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	15
Guarantee Duration, 0 - 3 Years [Member] | Financial And Performance Guarantee Other [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	38
Guarantee Duration, 4 - 10 Years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	7
Guarantee Duration, 4 - 10 Years [Member] | Financial And Performance Guarantee For Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	2
Guarantee Duration, 4 - 10 Years [Member] | Financial And Performance Guarantee Other [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	5
Guarantee Duration, Greater Than 10 Years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	67
Guarantee Duration, Greater Than 10 Years [Member] | Financial And Performance Guarantee For Energy Transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	66
Guarantee Duration, Greater Than 10 Years [Member] | Financial And Performance Guarantee For Intra-Company [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 1