Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,300	$ 1,234	$ 1,374
Interest income recognized on TDR loans	45	48	49
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	256	265	441
Interest income recognized on TDR loans	7	11	15
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	136	224	349
Interest income recognized on TDR loans	4	10	12
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	102	24	44
Interest income recognized on TDR loans	3	1	0
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	18	10	20
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	7	28
Interest income recognized on TDR loans	0	0	3
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,044	969	933
Interest income recognized on TDR loans	38	37	34
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,017	942	902
Interest income recognized on TDR loans	37	36	32
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	21	19	20
Interest income recognized on TDR loans	1	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	6	8	11
Interest income recognized on TDR loans	$ 0	$ 0	$ 1