Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2019		Dec. 31, 2018		Mar. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 787		$ 1,012
Current	[1]	70,247		67,966
Total loans		71,034		68,978		$ 67,338
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		56		211
Current	[1]	51,351		49,131
Total loans		51,407		49,342
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		5		79
Current	[1]	11,285		11,265
Total loans		11,290		11,344		11,054
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		51		117
Current	[1]	13,599		12,949
Total loans		13,650		13,066		12,818
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		0
Current	[1]	21,417		20,167
Total loans		21,417	[2]	20,167	[2]	19,289
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		15
Current	[1]	5,050		4,750
Total loans		5,050	[3]	4,765	[3]	4,578
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		731		801
Current	[1]	18,896		18,835
Total loans		19,627		19,636
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		650		723
Current	[1]	16,718		16,660
Total loans		17,368		17,383		17,325
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		40		41
Current	[1]	901		941
Total loans		941		982		1,139
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		31		27
Current	[1]	1,009		992
Total loans		1,040		1,019		770
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		10		10
Current	[1]	268		242
Total loans		278		252		$ 365
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		447		671
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		20		170
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		3		76
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		17		79
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		0
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		15
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		427		501
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		391		469
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		15		14
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		16		13
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		5		5
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		340		341
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		36		41
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		2		3
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		34		38
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0		0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		304		300
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		259		254
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		25		27
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		15		14
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 5		$ 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] 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.