Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,480	$ 1,225
Interest income recognized on TDR loans	12	11
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	404	223
Interest income recognized on TDR loans	2	2
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	93	182
Interest income recognized on TDR loans	1	1
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	260	34
Interest income recognized on TDR loans	1	1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	51	7
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,076	1,002
Interest income recognized on TDR loans	10	9
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,047	976
Interest income recognized on TDR loans	10	9
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	24	20
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 5	$ 6