Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,025	$ 1,183
Interest income recognized on TDR loans	12	10
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	311	435
Interest income recognized on TDR loans	5	3
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	31
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	175	285
Interest income recognized on TDR loans	4	3
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	136	119
Interest income recognized on TDR loans	1	0
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	714	748
Interest income recognized on TDR loans	7	7
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	676	715
Interest income recognized on TDR loans	7	7
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	34	29
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 4	$ 4