Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 448	$ 894	$ 921
Total interest income recognized on impaired commercial loans	22	11	13
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	9	32	83
Total interest income recognized on impaired commercial loans	0	0	4
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	200	304	344
Total interest income recognized on impaired commercial loans	15	9	9
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	239	554	487
Total interest income recognized on impaired commercial loans	7	2	0
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 0	$ 4	$ 7