Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) (Commercial Loans [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 306	$ 397
Total interest income recognized on impaired commercial loans	2	6
Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	198	297
Total interest income recognized on impaired commercial loans	1	6
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	93	35
Total interest income recognized on impaired commercial loans	1	0
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	7	33
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 8	$ 32