Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 1,333	$ 1,546
Current	[1]	83,609	76,195
Total loans		84,942	77,741	$ 73,439
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		63	109
Current	[1]	64,689	57,978
Total loans		64,752	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		20	47
Current	[1]	12,076	10,253
Total loans		12,096	10,300	9,661
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	41
Current	[1]	19,480	17,778
Total loans		19,514	17,819	17,355
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]	29,808	26,387
Total loans		29,808	26,387	24,001
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		9	21
Current	[1]	3,325	3,560
Total loans		3,334	3,581	3,045
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		1,270	1,437
Current	[1]	18,920	18,217
Total loans		20,190	19,654
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		1,173	1,321
Current	[1]	16,287	15,340
Total loans		17,460	16,661	16,402
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		63	77
Current	[1]	1,576	1,707
Total loans		1,639	1,784	1,824
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		19	21
Current	[1]	663	699
Total loans		682	720	686
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		15	18
Current	[1]	394	471
Total loans		409	489	$ 465
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		432	503
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		28	72
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		15	25
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		10	32
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		3	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		404	431
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		373	390
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		14	21
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	11
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		7	9
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		901	1,043
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		35	37
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Days Past Due, 30 Days or More		5	22
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		24	9
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		6	6
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		866	1,006
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		800	931
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		49	56
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		9	10
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		$ 8	$ 9

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