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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 287	$ 258	$ 296	$ 326
Total interest income recognized on impaired commercial loans	3	2	6	9
Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	124	208	161	252
Total interest income recognized on impaired commercial loans	1	2	3	9
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	139	40	116	37
Total interest income recognized on impaired commercial loans	2	0	3	0
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	17	0	12	16
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 7	$ 10	$ 7	$ 21