Employee Benefits - Change in FVA (Other Post Retirement Benefit Plan Assets) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
FVA at end of year	$ 942	$ 918
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
FVA at beginning of year	57	61
Employer contributions	(1)	1
Plan participants' contributions	2	5
Benefit payments	(7)	(8)
Transfer to insurer	(7)
Actual return on plan assets	7	(2)
FVA at end of year	$ 51	$ 57