Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 15		$ 18
Total loans	67,338		72,563		$ 69,588
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	47,188		52,421
Nonaccrual Loans	550		613
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	47,739		53,035
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	11,041		10,521
Nonaccrual Loans	13		12
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	11,054		10,533		10,802
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	12,559		12,288
Nonaccrual Loans	258		215
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	12,818		12,504		13,152
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	19,010		19,703
Nonaccrual Loans	279		385
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	19,289	[1],[2]	20,088	[1],[2]	22,527
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	4,578		9,909
Nonaccrual Loans	0		1
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	$ 4,578	[2]	$ 9,910	[2]	$ 3,515

[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] ncludes loans to HSBC affiliates which totaled $1,920 million and $6,750 million at March 31, 2018 and December 31, 2017, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.