Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Receivables [Abstract]
TDR Loans Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell	$ 763	$ 706	$ 608