Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 945	$ 1,158	$ 1,252
Interest income recognized on TDR loans	44	39	39
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	246	419	482
Interest income recognized on TDR loans	15	9	13
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	19	69
Interest income recognized on TDR loans	0	0	4
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	122	246	292
Interest income recognized on TDR loans	12	7	8
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	124	154	121
Interest income recognized on TDR loans	3	2	1
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	699	739	770
Interest income recognized on TDR loans	29	30	26
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	660	704	740
Interest income recognized on TDR loans	27	28	25
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	35	31	25
Interest income recognized on TDR loans	2	2	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 4	$ 4	$ 5