Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 18		$ 18
Total loans	72,563		73,875		$ 82,917
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	52,421		53,425
Nonaccrual Loans	613		790
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	53,035		54,216
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	10,521		10,834
Nonaccrual Loans	12		56
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	10,533		10,890		10,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	12,288		13,892
Nonaccrual Loans	215		187
Accruing Loans Contractually Past Due 90 days or More	1		1
Total loans	12,504		14,080		14,365
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	19,703		22,935
Nonaccrual Loans	385		546
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	20,088	[1],[2]	23,481	[1],[2]	29,905
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	9,909		5,764
Nonaccrual Loans	1		1
Accruing Loans Contractually Past Due 90 days or More	0		0
Total loans	$ 9,910	[1]	$ 5,765	[1]	$ 8,183

[1] During 2017, in conjunction with the creation of the new Corporate Center segment as discussed further in Note 22, "Business Segments," we reclassified loans to HSBC affiliates from global banking to other commercial and revised prior periods to conform with the current year presentation. As a result, other commercial includes loans to HSBC affiliates which totaled $6,750 million and $3,274 million at December 31, 2017 and 2016, respectively. All tables below have been restated to reflect this reclassification, as applicable. See Note 21, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.

[2] 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.