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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 532	$ 932	$ 586	$ 972
Total interest income recognized on impaired commercial loans	2	2	8	6
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	12	44	11	43
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	272	304	264	317
Total interest income recognized on impaired commercial loans	1	1	6	5
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	248	577	311	605
Total interest income recognized on impaired commercial loans	1	1	2	1
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 0	$ 7	$ 0	$ 7