Loans - Credit Risk Profile of Commercial Loans (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 67,710		$ 63,258		$ 61,452
Commercial Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	48,509		44,150
Commercial Loan [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,919		8,457		7,920
Commercial Loan [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,668		12,608		12,497
Commercial Loan [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,370		20,009		19,011
Commercial Loan [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,552		3,076		3,140
Commercial Loan [Member] | External Credit Rating, Investment Grade [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	33,242	[1]	28,198	[1]
Commercial Loan [Member] | External Credit Rating, Investment Grade [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,583	[1]	4,727	[1]
Commercial Loan [Member] | External Credit Rating, Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,953	[1]	6,012	[1]
Commercial Loan [Member] | External Credit Rating, Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,518	[1]	16,206	[1]
Commercial Loan [Member] | External Credit Rating, Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,188	[1]	1,253	[1]
Commercial Loan [Member] | External Credit Rating, Non Investment Grade [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	15,267		15,952
Commercial Loan [Member] | External Credit Rating, Non Investment Grade [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,336		3,730
Commercial Loan [Member] | External Credit Rating, Non Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,715		6,596
Commercial Loan [Member] | External Credit Rating, Non Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,852		3,803
Commercial Loan [Member] | External Credit Rating, Non Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 1,364		$ 1,823

[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.