Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions	Mar. 31, 2019		Dec. 31, 2018		Mar. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 71,034		$ 68,978		$ 67,338
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	51,407		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,290		11,344		11,054
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,650		13,066		12,818
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	21,417	[1]	20,167	[1]	19,289
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,050	[2]	4,765	[2]	$ 4,578
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,871		1,611
Commercial Loans [Member] | Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	667		549
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	694		522
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	510		539
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans			1
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,151		907
Commercial Loans [Member] | Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	576		452
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	335		193
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	240		262
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans			0
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	698		685
Commercial Loans [Member] | Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	88		93
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	340		314
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	270		277
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans			1
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	22		19
Commercial Loans [Member] | Doubtful [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3		4
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19		15
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0		0
Commercial Loans [Member] | Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans			$ 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 $2,688 million and $2,274 million at March 31, 2019 and December 31, 2018, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.