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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
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]      
Loans $ 67,338 $ 72,563 $ 69,588
TDR loans carried at lower of amortized cost or fair value less cost to sell 644 655 681
Affiliated Entity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Loans $ 1,920 $ 6,750 $ 939