Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other loans included in impaired loans apart from TDR loans	$ 64	$ 329
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell	623	655	$ 662
Non Accrual loans included in TDR loan	340	559
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional commitments to lend to commercial borrowers with modified TDRs	$ 105	$ 245