Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 71,336		$ 68,978		$ 66,756
TDR loans carried at lower of amortized cost or fair value less cost to sell	571		615		623
Affiliated Entity [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	2,828		2,274
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	51,317		49,342
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	5,712	[1]	$ 4,765	[1]	3,898
Commercial Loans [Member] | Other Commercial [Member] | Affiliated Entity [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 2,828				$ 1,557

[1] See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.