Employee Benefits - Net Actuarial Loss and Prior Service Cost Included in Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 989	$ 928
Prior service cost	18	20
Total net actuarial loss and prior service cost included in accumulated other comprehensive income	1,007	948
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	108	122
Prior service cost
Total net actuarial loss and prior service cost included in accumulated other comprehensive income	$ 108	$ 122