Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 950	$ 1,181	$ 996	$ 1,198
Interest income recognized on TDR loans	9	10	21	20
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	247	439	288	453
Interest income recognized on TDR loans	1	2	6	5
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	31	0	31
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	137	265	156	277
Interest income recognized on TDR loans	0	1	4	4
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	110	143	132	145
Interest income recognized on TDR loans	1	1	2	1
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	703	742	708	745
Interest income recognized on TDR loans	8	8	15	15
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	664	708	670	712
Interest income recognized on TDR loans	7	7	14	14
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	35	30	34	29
Interest income recognized on TDR loans	1	1	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 4	$ 4	$ 4	$ 4