Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 1,194	$ 287	$ 888	$ 296
Total interest income recognized on impaired commercial loans	5	3	11	6
Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	79	124	93	161
Total interest income recognized on impaired commercial loans	2	1	3	3
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	474	139	398	116
Total interest income recognized on impaired commercial loans	3	2	8	3
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	633	17	390	12
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 8	$ 7	$ 7	$ 7