Loans - Status of Commercial Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 20	$ 21
Total loans	81,458	77,741	68,897
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	61,494	57,947
Nonaccrual loans	152	139
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	61,647	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	10,827	10,235
Nonaccrual loans	74	65
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	10,901	10,300	9,165
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	19,172	17,744
Nonaccrual loans	78	74
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	19,251	17,819	15,698
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	28,029	26,387
Nonaccrual loans	0	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	28,029	26,387	21,979
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	3,466	3,581
Nonaccrual loans	0	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	$ 3,466	$ 3,581	$ 3,051