Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Financing Receivable, Credit Quality Indicator [Line Items]
Nonaccrual Loans	$ 664		$ 539
Accruing Loans Contractually Past Due 90 days or More	30		21
Total loans	68,553		68,978		$ 72,563
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	47,973		49,198
Nonaccrual Loans	237		143
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	48,211		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	11,495		11,337
Nonaccrual Loans	6		7
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	11,501		11,344		10,533
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	13,396		12,995
Nonaccrual Loans	82		70
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	13,479		13,066		12,504
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	17,766		20,102
Nonaccrual Loans	149		65
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	17,915	[1]	20,167	[1]	20,088
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans	5,316		4,764
Nonaccrual Loans	0		1
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	$ 5,316	[2]	$ 4,765	[2]	$ 9,910

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