Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	$ 760	$ 910	$ 795	$ 967
Interest income recognized on TDR loans	8	12	25	33
Commercial Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	121	214	143	262
Interest income recognized on TDR loans	1	6	4	12
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	50	97	60	136
Interest income recognized on TDR loans	0	5	2	9
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	71	117	83	126
Interest income recognized on TDR loans	1	1	2	3
Consumer Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	639	696	652	705
Interest income recognized on TDR loans	7	6	21	21
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	602	657	615	666
Interest income recognized on TDR loans	7	6	20	20
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	33	35	33	35
Interest income recognized on TDR loans	0	0	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	$ 4	$ 4	$ 4	$ 4