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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
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2016
Sep. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]              
Loans $ 77,625 $ 77,625 $ 67,593   $ 73,875    
Allowance for credit losses 1,060 1,060 799 $ 848 1,017 $ 1,063 $ 912
TDR loans carried at lower of amortized cost or fair value less cost to sell 662 662 662   672    
Affiliated Entity [Member]              
Financing Receivable, Allowance for Credit Losses [Line Items]              
Loans 4,463 4,463 1,820   3,274    
Commercial Loan [Member]              
Financing Receivable, Allowance for Credit Losses [Line Items]              
Loans     48,128   54,216    
Consumer Loans [Member]              
Financing Receivable, Allowance for Credit Losses [Line Items]              
Loans     19,465   19,659    
Loans Reclassified to Global Banking [Member] | Commercial Loan [Member]              
Financing Receivable, Allowance for Credit Losses [Line Items]              
Loans 3,600 3,600          
Allowance for credit losses 21 21          
Residential Mortgage [Member] | Consumer Loans [Member]              
Financing Receivable, Allowance for Credit Losses [Line Items]              
Loans 17,091 17,091 17,196   17,181    
Allowance for credit losses 30 30 $ 26 $ 20 $ 26 $ 50 $ 68
Provision for credit losses and charge-offs related to lower of amortized cost or fair value adjustment attributable to credit factors for loans transferred to held for sale 11 11          
Additional charge-off for the existing allowance for credit losses on collateral dependent loans transferred to held for sale $ 20 $ 22