Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 68,553		$ 68,978		$ 72,563
TDR loans carried at lower of amortized cost or fair value less cost to sell	557		615		655
Affiliated Entity [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	2,343		2,274
Other Commercial [Member] | Affiliated Entity [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans			2,274
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	48,211		49,342
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	5,316	[1]	4,765	[1]	9,910
Commercial Loans [Member] | Other Commercial [Member] | Affiliated Entity [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 2,343		$ 2,274		$ 6,750

[1] See Note 23, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.