Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 894	$ 1,194	$ 943	$ 888
Total interest income recognized on impaired commercial loans	2	5	8	11
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	30	79	37	93
Total interest income recognized on impaired commercial loans	0	2	0	3
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	302	428	318	345
Total interest income recognized on impaired commercial loans	1	3	6	8
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	559	679	583	443
Total interest income recognized on impaired commercial loans	1	0	2	0
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 3	$ 8	$ 5	$ 7