Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions	Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	$ 823		$ 923
Current	66,515		71,640
Total loans	67,338		72,563		$ 69,588
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	102		134
Current	47,637		52,901
Total loans	47,739		53,035
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	52		36
Current	11,002		10,497
Total loans	11,054		10,533		10,802
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	38		30
Current	12,780		12,474
Total loans	12,818		12,504		13,152
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	0		25
Current	19,289		20,063
Total loans	19,289	[1],[2]	20,088	[1],[2]	22,527
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	12		43
Current	4,566		9,867
Total loans	4,578	[2]	9,910	[2]	3,515
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	721		789
Current	18,878		18,739
Total loans	19,599		19,528
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	649		713
Current	16,676		16,560
Total loans	17,325		17,273		17,219
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	44		47
Current	1,095		1,144
Total loans	1,139		1,191		1,344
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	18		17
Current	752		704
Total loans	770		721		646
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More	10		12
Current	355		331
Total loans	365		343		$ 383
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	452		488
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	90		95
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	42		27
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	36		25
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	12		43
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	362		393
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	337		369
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	10		11
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	10		8
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	371		435
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	12		39
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	10		9
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	2		5
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		25
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	359		396
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	312		344
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	34		36
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	8		9
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		$ 7

[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 $1,920 million and $6,750 million at March 31, 2018 and December 31, 2017, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.