Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2019		Dec. 31, 2018		Mar. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 21		$ 21
Total loans	71,034		68,978		$ 67,338
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	51,275		49,198
Nonaccrual Loans	131		143
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	51,407		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	11,285		11,337
Nonaccrual Loans	5		7
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	11,290		11,344		11,054
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	13,571		12,995
Nonaccrual Loans	78		70
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	13,650		13,066		12,818
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	21,369		20,102
Nonaccrual Loans	48		65
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	21,417	[1]	20,167	[1]	19,289
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	5,050		4,764
Nonaccrual Loans	0		1
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	$ 5,050	[2]	$ 4,765	[2]	$ 4,578

[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] Includes loans to HSBC affiliates which totaled $2,688 million and $2,274 million at March 31, 2019 and December 31, 2018, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.