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Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product - Additional Information (Detail) - USD ($)
$ in Millions
Jun. 30, 2015
Dec. 31, 2014
Jun. 30, 2014
Receivables [Abstract]      
TDR Loans Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell $ 775 $ 763 $ 725