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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 346	$ 894	$ 503	$ 943
Total interest income recognized on impaired commercial loans	7	2	15	8
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	8	30	10	37
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	185	302	227	318
Total interest income recognized on impaired commercial loans	6	1	12	6
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	152	559	266	583
Total interest income recognized on impaired commercial loans	1	1	3	2
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 1	$ 3	$ 0	$ 5