Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 25		$ 21
Total loans	71,336		68,978		$ 66,756
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	51,052		49,198
Nonaccrual Loans	264		143
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	51,317		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	11,893		11,337
Nonaccrual Loans	3		7
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	11,896		11,344		11,882
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	14,682		12,995
Nonaccrual Loans	149		70
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	14,832		13,066		12,731
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	18,765		20,102
Nonaccrual Loans	112		65
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	18,877	[1]	20,167	[1]	18,672
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	5,712		4,764
Nonaccrual Loans	0		1
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	$ 5,712	[2]	$ 4,765	[2]	$ 3,898

[1] Represents large multinational firms including globally focused U.S. corporate and financial institutions, U.S. dollar lending to multinational banking clients managed by HSBC on a global basis and complex large business clients supported by Global Banking and Markets relationship managers.

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