Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,300	$ 1,207	$ 1,259	$ 1,241
Interest income recognized on TDR loans	11	11	33	36
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	235	231	229	279
Interest income recognized on TDR loans	2	2	5	9
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	112	202	147	233
Interest income recognized on TDR loans	1	2	3	9
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	106	29	70	24
Interest income recognized on TDR loans	1	0	2	0
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	17	0	12	13
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	0	0	9
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,065	976	1,030	962
Interest income recognized on TDR loans	9	9	28	27
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,037	949	1,004	935
Interest income recognized on TDR loans	9	9	27	26
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	22	19	20	19
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	$ 6	$ 8	$ 6	$ 8