Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 20	$ 21
Total loans	84,942	77,741	$ 73,439
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	64,607	57,947
Nonaccrual Loans	144	139
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	64,752	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	12,047	10,235
Nonaccrual Loans	49	65
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	12,096	10,300	9,661
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	19,439	17,744
Nonaccrual Loans	74	74
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	19,514	17,819	17,355
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	29,787	26,387
Nonaccrual Loans	21	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	29,808	26,387	24,001
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	3,334	3,581
Nonaccrual Loans	0	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	$ 3,334	$ 3,581	$ 3,045