Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 991	$ 515
Total interest income recognized on impaired commercial loans	4	3
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	44	107
Total interest income recognized on impaired commercial loans	0	1
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	333	262
Total interest income recognized on impaired commercial loans	4	2
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	607	139
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 7	$ 7