Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 781		$ 1,012
Current	[1]	70,555		67,966
Total loans		71,336		68,978		$ 66,756
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		141		211
Current	[1]	51,176		49,131
Total loans		51,317		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		10		79
Current	[1]	11,886		11,265
Total loans		11,896		11,344		11,882
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		54		117
Current	[1]	14,778		12,949
Total loans		14,832		13,066		12,731
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		70		0
Current	[1]	18,807		20,167
Total loans		18,877	[2]	20,167	[2]	18,672
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		7		15
Current	[1]	5,705		4,750
Total loans		5,712	[3]	4,765	[3]	3,898
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		640		801
Current	[1]	19,379		18,835
Total loans		20,019		19,636
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		557		723
Current	[1]	17,017		16,660
Total loans		17,574		17,383		17,376
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		35		41
Current	[1]	845		941
Total loans		880		982		1,027
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		37		27
Current	[1]	1,271		992
Total loans		1,308		1,019		907
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		11		10
Current	[1]	246		242
Total loans		257		252		$ 263
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		464		671
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		125		170
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		8		76
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		40		79
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		70		0
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		7		15
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		339		501
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		304		469
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		11		14
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		19		13
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		5		5
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		317		341
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		16		41
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		2		3
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		14		38
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		301		300
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		253		254
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		24		27
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		18		14
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 6		$ 5

[1] Loans less than 30 days past due are presented as current.

[2] 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.

[3] See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.