Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 19	$ 21
Total loans	83,617	77,741	$ 72,198
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	63,484	57,947
Nonaccrual loans	111	139
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	63,596	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	11,456	10,235
Nonaccrual loans	64	65
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	11,520	10,300	9,682
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	19,664	17,744
Nonaccrual loans	47	74
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	19,712	17,819	15,956
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	28,942	26,387
Nonaccrual loans	0	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	28,942	26,387	24,268
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	3,422	3,581
Nonaccrual loans	0	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	$ 3,422	$ 3,581	$ 3,127