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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
Mar. 31, 2013
Mar. 31, 2012
Income Tax Disclosure [Abstract]    
Tax at statutory rate $ 681 $ 641
State income tax, at statutory rates, net of federal tax benefit 36 36
Tax effect of    
Tax credits, net of related expenses (106) (89)
Tax-exempt income (54) (55)
Noncontrolling interests 14 11
Other items (13) (17)
Total income tax provision $ 558 $ 527