Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,158	$ 1,252	$ 1,300
Interest income recognized on TDR loans	39	39	45
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	419	482	256
Interest income recognized on TDR loans	9	13	7
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	19	69	136
Interest income recognized on TDR loans	0	4	4
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	246	292	76
Interest income recognized on TDR loans	7	8	3
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	154	121	44
Interest income recognized on TDR loans	2	1	0
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	739	770	1,044
Interest income recognized on TDR loans	30	26	38
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	704	740	1,017
Interest income recognized on TDR loans	28	25	37
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	31	25	21
Interest income recognized on TDR loans	2	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 4	$ 5	$ 6