Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 660	$ 991
Total interest income recognized on impaired commercial loans	6	4
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	12	44
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	268	333
Total interest income recognized on impaired commercial loans	5	4
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	380	607
Total interest income recognized on impaired commercial loans	1	0
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 0	$ 7