Loans - Credit Risk Profile of Commercial Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 72,563		$ 73,875		$ 82,917
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		53,035		54,216
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		10,533		10,890		10,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		12,504		14,080		14,365
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		20,088	[1],[2]	23,481	[1],[2]	29,905
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		9,910	[1]	5,765	[1]	$ 8,183
Investment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	34,767		32,883
Investment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	7,456		7,857
Investment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	5,752		6,348
Investment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	13,218		14,205
Investment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	8,341		4,473
Noninvestment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		18,268		21,333
Noninvestment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		3,077		3,033
Noninvestment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		6,752		7,732
Noninvestment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		6,870		9,276
Noninvestment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 1,569		$ 1,292

[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.

[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.