Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,183	$ 1,480
Interest income recognized on TDR loans	10	12
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	435	404
Interest income recognized on TDR loans	3	2
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	31	93
Interest income recognized on TDR loans	0	1
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	285	199
Interest income recognized on TDR loans	3	1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	119	112
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	748	1,076
Interest income recognized on TDR loans	7	10
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	715	1,047
Interest income recognized on TDR loans	7	10
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	29	24
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 4	$ 5