Loans - Credit Risk Profile of Commercial Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		$ 71,336		$ 68,978		$ 66,756
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		51,317		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		11,896		11,344		11,882
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		14,832		13,066		12,731
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		18,877	[1]	20,167	[1]	18,672
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		5,712	[2]	4,765	[2]	$ 3,898
Investment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	31,387		31,197
Investment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	6,610		6,769
Investment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	6,261		5,674
Investment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	13,585		14,764
Investment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	[3]	4,931		3,990
Noninvestment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		19,930		18,145
Noninvestment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		5,286		4,575
Noninvestment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		8,571		7,392
Noninvestment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		5,292		5,403
Noninvestment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		$ 781		$ 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 14, "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.