Other Material Contingencies - Total Guarantees (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Guarantor Obligations [Line Items]
Total guarantees, by type and term	$ 878	$ 859
Energy transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	826
Intra-company guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	16
Other guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	36
0 - 3 years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	815
0 - 3 years [Member] | Energy transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	763
0 - 3 years [Member] | Intra-company guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	16
0 - 3 years [Member] | Other guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	36
4 - 10 years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	31
4 - 10 years [Member] | Energy transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	31
4 - 10 years [Member] | Intra-company guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term
4 - 10 years [Member] | Other guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term
Greater than 10 years [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	32
Greater than 10 years [Member] | Energy transactions [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term	32
Greater than 10 years [Member] | Intra-company guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term
Greater than 10 years [Member] | Other guarantees [Member]
Guarantor Obligations [Line Items]
Total guarantees, by type and term