Loans - Status of Commercial Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 21	$ 34
Total loans	77,741	67,695	63,258
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	57,947	48,234
Nonaccrual loans	139	254
Accruing Loans Contractually Past Due 90 days or More	1	6
Total loans	58,087	48,494
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	10,235	8,868
Nonaccrual loans	65	166
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	10,300	9,034	8,457
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	17,744	14,420
Nonaccrual loans	74	21
Accruing Loans Contractually Past Due 90 days or More	1	5
Total loans	17,819	14,446	12,608
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	26,387	21,560
Nonaccrual loans	0	65
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	26,387	21,625	20,009
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	3,581	3,386
Nonaccrual loans	0	2
Accruing Loans Contractually Past Due 90 days or More	0	1
Total loans	$ 3,581	$ 3,389	$ 3,076