Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions	Jun. 30, 2018		Dec. 31, 2017		Jun. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 66,399		$ 72,563		$ 68,063
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	46,843		53,035
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,245		10,533		10,240
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,109		12,504		12,868
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	18,877	[1],[2]	20,088	[1],[2]	21,047
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,612	[2]	9,910	[2]	$ 4,433
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,676		3,782
Commercial Loans [Member] | Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	569		585
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	727		1,040
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,370		2,146
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10		11
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,155		1,407
Commercial Loans [Member] | Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	480		467
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	368		477
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	297		452
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10		11
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,478		2,248
Commercial Loans [Member] | Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	85		117
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	334		519
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,059		1,612
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0		0
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	43		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	25		44
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	14		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] ncludes loans to HSBC affiliates which totaled $1,895 million and $6,750 million at June 30, 2018 and December 31, 2017, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.