Loans - Average Balance and Interest Income Recognized on Impaired Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 894	$ 921	$ 327
Total interest income recognized on impaired commercial loans	11	13	8
Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	32	83	151
Total interest income recognized on impaired commercial loans	0	4	4
Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	304	344	125
Total interest income recognized on impaired commercial loans	9	9	4
Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	554	487	44
Total interest income recognized on impaired commercial loans	2	0	0
Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Total average balance of impaired commercial loans	$ 4	$ 7	$ 7