Loans - Status of Commercial Loan Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Accruing Loans Contractually Past Due 90 days or More	$ 17	$ 21
Total loans	82,917	77,741	$ 67,695
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	62,187	57,947
Nonaccrual Loans	265	139
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	62,453	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	9,947	10,235
Nonaccrual Loans	53	65
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	10,000	10,300	9,034
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	18,948	17,744
Nonaccrual Loans	167	74
Accruing Loans Contractually Past Due 90 days or More	1	1
Total loans	19,116	17,819	14,446
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	29,925	26,387
Nonaccrual Loans	44	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	29,969	26,387	21,625
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing Loans	3,367	3,581
Nonaccrual Loans	1	0
Accruing Loans Contractually Past Due 90 days or More	0	0
Total loans	$ 3,368	$ 3,581	$ 3,389