Loans - Credit Risk Profile of Commercial Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 82,917	$ 77,741	$ 67,695
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		62,453	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		10,000	10,300	9,034
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		19,116	17,819	14,446
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		29,969	26,387	21,625
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		3,368	3,581	$ 3,389
Internal Investment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	43,854	41,928
Internal Investment Grade [Member] | Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	8,487	7,820
Internal Investment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	10,373	8,835
Internal Investment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	23,111	23,400
Internal Investment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	1,883	1,873
Internal Noninvestment Grade [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		18,599	16,159
Internal Noninvestment Grade [Member] | Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		1,513	2,480
Internal Noninvestment Grade [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		8,743	8,984
Internal Noninvestment Grade [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		6,858	2,987
Internal Noninvestment Grade [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 1,485	$ 1,708

[1] Investment grade includes commercial loans with credit ratings of at least BBB- or above or the equivalent based on our internal credit rating system.