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Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans - Additional Information (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Other loans included in impaired loans apart from TDR loans $ 200 $ 329  
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell 635 655 $ 670
Non Accrual loans included in TDR loan 433 559  
Commercial Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Additional commitments to lend to commercial borrowers with modified TDRs $ 121 $ 245