Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 932	$ 832	$ 972	$ 705
Total interest income recognized on impaired commercial loans	2	5	6	8
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	44	101	43	104
Total interest income recognized on impaired commercial loans	0	0	0	1
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	304	381	317	326
Total interest income recognized on impaired commercial loans	1	3	5	5
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	577	342	605	268
Total interest income recognized on impaired commercial loans	1	2	1	2
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 7	$ 8	$ 7	$ 7