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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 209	$ 532	$ 217	$ 586
Total interest income recognized on impaired commercial loans	2	2	4	8
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	4	12	5	11
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	105	272	101	264
Total interest income recognized on impaired commercial loans	2	1	3	6
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	100	248	111	311
Total interest income recognized on impaired commercial loans	$ 0	$ 1	$ 1	$ 2