Loans - Credit Risk Profile of Commercial Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 68,978		$ 72,563		$ 73,875
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		49,342		53,035
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		11,344		10,533		10,890
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		13,066		12,504		14,080
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		20,167	[1]	20,088	[1]	23,481
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		4,765	[2]	9,910	[2]	$ 5,765
Investment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	31,197		34,767
Investment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	6,769		7,456
Investment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	5,674		5,752
Investment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	14,764		13,218
Investment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[3]	3,990		8,341
Noninvestment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		18,145		18,268
Noninvestment Grade [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		4,575		3,077
Noninvestment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		7,392		6,752
Noninvestment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		5,403		6,870
Noninvestment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 775		$ 1,569

[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 $2,274 million and $6,750 million at December 31, 2018 and 2017, respectively. See Note 22, "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.