Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) (Commercial Loans [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 320	$ 586	$ 893
Total interest income recognized on impaired commercial loans	12	18	20
Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	241	422	602
Total interest income recognized on impaired commercial loans	10	13	11
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	48	63	119
Total interest income recognized on impaired commercial loans	2	0	5
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	13	37	86
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	18	64	86
Total interest income recognized on impaired commercial loans	$ 0	$ 5	$ 4