Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	$ 799	$ 950	$ 817	$ 996
Interest income recognized on TDR loans	8	9	17	21
Commercial Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	147	247	158	288
Interest income recognized on TDR loans	1	1	3	6
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	65	137	66	156
Interest income recognized on TDR loans	1	0	2	4
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	82	110	92	132
Interest income recognized on TDR loans	0	1	1	2
Consumer Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	652	703	659	708
Interest income recognized on TDR loans	7	8	14	15
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	615	664	621	670
Interest income recognized on TDR loans	6	7	13	14
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	33	35	34	34
Interest income recognized on TDR loans	1	1	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average balance of TDR loans	$ 4	$ 4	$ 4	$ 4