Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,181	$ 1,484	$ 1,198	$ 1,487
Interest income recognized on TDR loans	10	13	20	25
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	439	520	453	468
Interest income recognized on TDR loans	2	3	5	5
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	31	87	31	89
Interest income recognized on TDR loans	0	0	0	1
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	265	326	277	268
Interest income recognized on TDR loans	1	3	4	4
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	143	107	145	111
Interest income recognized on TDR loans	1	0	1	0
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	742	964	745	1,019
Interest income recognized on TDR loans	8	10	15	20
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	708	934	712	990
Interest income recognized on TDR loans	7	9	14	19
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	30	25	29	24
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	$ 5	$ 4	$ 5