Employee Benefits - Pre-tax AOCI Not Yet Recognized as Net Postretirement Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Total unrecognized AOCI	$ 681	$ 634
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Net unrecognized losses (gains)	5	8
Net unrecognized prior service benefit	(7)	(8)
Total unrecognized AOCI	$ (2)