Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 910	$ 1,167	$ 967	$ 1,174
Interest income recognized on TDR loans	12	9	33	29
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	214	427	262	431
Interest income recognized on TDR loans	6	2	12	7
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	15	0	23
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	97	248	136	259
Interest income recognized on TDR loans	5	1	9	5
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	117	164	126	149
Interest income recognized on TDR loans	1	1	3	2
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	696	740	705	743
Interest income recognized on TDR loans	6	7	21	22
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	657	705	666	709
Interest income recognized on TDR loans	6	7	20	21
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	35	31	35	30
Interest income recognized on TDR loans	0	0	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 4	$ 4	$ 4	$ 4