Loans - Summary of Criticized Assets for 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
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,611		3,782
Commercial Loans [Member] | Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	549		585
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	522		1,040
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	539		2,146
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1		11
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	907		1,407
Commercial Loans [Member] | Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	452		467
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	193		477
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	262		452
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0		11
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	685		2,248
Commercial Loans [Member] | Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	93		117
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	314		519
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	277		1,612
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1		0
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19		127
Commercial Loans [Member] | Doubtful [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4		1
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	15		44
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0		82
Commercial Loans [Member] | Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0		$ 0

[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.