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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 251	$ 346	$ 237	$ 503
Total interest income recognized on impaired commercial loans	2	7	6	15
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	4	8	5	10
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	130	185	116	227
Total interest income recognized on impaired commercial loans	1	6	4	12
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	117	152	116	266
Total interest income recognized on impaired commercial loans	1	1	2	3
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 0	$ 1	$ 0	$ 0