Income Taxes - Reconciliation of Expected Income Tax Expense at Federal Statutory Rate of 35 Percent to Company's Applicable Income Tax Expense (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 690	$ 709	$ 2,061	$ 2,030
State income tax, at statutory rates, net of federal tax benefit	62	53	102	157
Tax effect of
Tax credits, net of related expenses	(176)	(120)	(394)	(322)
Tax-exempt income	(52)	(55)	(159)	(165)
Noncontrolling interests	13	15	42	39
Other items	5	(9)	(23)	(55)
Total income tax provision	$ 542	$ 593	$ 1,629	$ 1,684