Depreciation Expense (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Supplemental Financial Information [Line Items]
Depreciation expense	$ 347	$ 319	$ 1,045	$ 961