Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,167	$ 1,508	$ 1,174	$ 1,477
Interest income recognized on TDR loans	9	15	29	40
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	427	559	431	481
Interest income recognized on TDR loans	2	5	7	10
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	15	63	23	78
Interest income recognized on TDR loans	0	2	0	3
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	248	380	259	290
Interest income recognized on TDR loans	1	3	5	7
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	164	116	149	113
Interest income recognized on TDR loans	1	0	2	0
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	740	949	743	996
Interest income recognized on TDR loans	7	10	22	30
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	705	917	709	966
Interest income recognized on TDR loans	7	10	21	29
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	31	27	30	25
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	$ 5	$ 4	$ 5