Loans - Summary of Past Due Status of Loans (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	$ 451		$ 503
Days Past Due, 90 + days	991		1,043
Total Days Past Due, 30 Days or More	1,442		1,546
Current	80,016	[1]	76,195	[1]
Total loans	81,458		77,741		68,897
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	59		72
Days Past Due, 90 + days	40		37
Total Days Past Due, 30 Days or More	99		109
Current	61,548	[1]	57,978	[1]
Total loans	61,647		58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	40		25
Days Past Due, 90 + days	30		22
Total Days Past Due, 30 Days or More	70		47
Current	10,831	[1]	10,253	[1]
Total loans	10,901		10,300		9,165
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	13		32
Days Past Due, 90 + days	4		9
Total Days Past Due, 30 Days or More	17		41
Current	19,234	[1]	17,778	[1]
Total loans	19,251		17,819		15,698
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	0		0
Days Past Due, 90 + days	0		0
Total Days Past Due, 30 Days or More	0		0
Current	28,029	[1]	26,387	[1]
Total loans	28,029		26,387		21,979
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	6		15
Days Past Due, 90 + days	6		6
Total Days Past Due, 30 Days or More	12		21
Current	3,454	[1]	3,560	[1]
Total loans	3,466		3,581		3,051
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	392		431
Days Past Due, 90 + days	951		1,006
Total Days Past Due, 30 Days or More	1,343		1,437
Current	18,468	[1]	18,217	[1]
Total loans	19,811		19,654
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	355		390
Days Past Due, 90 + days	879		931
Total Days Past Due, 30 Days or More	1,234		1,321
Current	15,757	[1]	15,340	[1]
Total loans	16,991		16,661		15,900
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	19		21
Days Past Due, 90 + days	54		56
Total Days Past Due, 30 Days or More	73		77
Current	1,643	[1]	1,707	[1]
Total loans	1,716		1,784		1,938
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	10		11
Days Past Due, 90 + days	10		10
Total Days Past Due, 30 Days or More	20		21
Current	660	[1]	699	[1]
Total loans	680		720		683
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 30-89 days	8		9
Days Past Due, 90 + days	8		9
Total Days Past Due, 30 Days or More	16		18
Current	408	[1]	471	[1]
Total loans	$ 424		$ 489		$ 483

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