Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 68,553		$ 68,978		$ 72,563
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	48,211		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	11,501		11,344		10,533
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	13,479		13,066		12,504
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	17,915	[1]	20,167	[1]	20,088
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	5,316	[2]	4,765	[2]	$ 9,910
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,001		1,611
Commercial Loans [Member] | Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	719		549
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	876		522
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	395		539
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	11		1
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,178		907
Commercial Loans [Member] | Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	516		452
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	467		193
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	184		262
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	11		0
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	785		685
Commercial Loans [Member] | Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	203		93
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	386		314
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	196		277
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0		1
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	38		19
Commercial Loans [Member] | Doubtful [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0		4
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	23		15
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	15		0
Commercial Loans [Member] | Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [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] See Note 23, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.