Loans - Credit Risk Profile of Commercial Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		$ 68,553		$ 68,978		$ 72,563
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		48,211		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		11,501		11,344		10,533
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		13,479		13,066		12,504
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		17,915	[1]	20,167	[1]	20,088
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		5,316	[2]	4,765	[2]	$ 9,910
Investment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	29,991		31,197
Investment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	6,332		6,769
Investment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	6,029		5,674
Investment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	12,981		14,764
Investment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	4,649		3,990
Noninvestment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		18,220		18,145
Noninvestment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		5,169		4,575
Noninvestment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		7,450		7,392
Noninvestment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		4,934		5,403
Noninvestment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		$ 667		$ 775

[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.

[3] Investment grade includes commercial loans with credit ratings of at least BBB- or above or the equivalent based on our internal credit rating system.