Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,508	$ 1,300	$ 1,477	$ 1,259
Interest income recognized on TDR loans	15	11	40	33
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	559	235	481	229
Interest income recognized on TDR loans	5	2	10	5
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	63	112	78	147
Interest income recognized on TDR loans	2	1	3	3
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	427	106	343	70
Interest income recognized on TDR loans	3	1	7	2
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	69	17	60	12
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	949	1,065	996	1,030
Interest income recognized on TDR loans	10	9	30	28
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	917	1,037	966	1,004
Interest income recognized on TDR loans	10	9	29	27
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	27	22	25	20
Interest income recognized on TDR loans	0	0	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 5	$ 6	$ 5	$ 6