Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	$ 781	$ 945	$ 1,158
Interest income recognized on TDR loans	32	44	39
Commercial Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	136	246	419
Interest income recognized on TDR loans	4	15	9
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	0	0	19
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	55	122	246
Interest income recognized on TDR loans	2	12	7
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	81	124	154
Interest income recognized on TDR loans	2	3	2
Consumer Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	645	699	739
Interest income recognized on TDR loans	28	29	30
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	608	660	704
Interest income recognized on TDR loans	26	27	28
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	33	35	31
Interest income recognized on TDR loans	2	2	2
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	$ 4	$ 4	$ 4