Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 515	$ 306
Total interest income recognized on impaired commercial loans	3	2
Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	107	198
Total interest income recognized on impaired commercial loans	1	1
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	323	93
Total interest income recognized on impaired commercial loans	2	1
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	78	7
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 7	$ 8