Loans - Credit Risk Profile of Commercial Loans (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 73,439		$ 67,695		$ 67,710
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	54,062		48,494
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,661		9,034		8,919
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	17,355		14,446		13,668
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	24,001		21,625		23,370
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,045		3,389		2,552
Commercial Loans [Member] | External Credit Rating, Investment Grade [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	38,594	[1]	33,567	[1]
Commercial Loans [Member] | External Credit Rating, Investment Grade [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,193	[1]	6,069	[1]
Commercial Loans [Member] | External Credit Rating, Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,635	[1]	7,279	[1]
Commercial Loans [Member] | External Credit Rating, Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	21,199	[1]	18,636	[1]
Commercial Loans [Member] | External Credit Rating, Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,567	[1]	1,583	[1]
Commercial Loans [Member] | External Credit Rating, Non Investment Grade [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	15,468		14,927
Commercial Loans [Member] | External Credit Rating, Non Investment Grade [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,468		2,965
Commercial Loans [Member] | External Credit Rating, Non Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,720		7,167
Commercial Loans [Member] | External Credit Rating, Non Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,802		2,989
Commercial Loans [Member] | External Credit Rating, Non Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 1,478		$ 1,806

[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.