Loans - Credit Risk Profile of Commercial Loans (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 62,932		$ 63,258		$ 53,869
Commercial Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	43,639		44,150
Commercial Loan [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,319		8,457		7,777
Commercial Loan [Member] | Business Banking and Middle Markets Enterprises [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,725		12,608		10,889
Commercial Loan [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,718		20,009		13,852
Commercial Loan [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,877		3,076		3,051
Commercial Loan [Member] | Investment Grade [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	27,204	[1]	28,198	[1]
Commercial Loan [Member] | Investment Grade [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,492	[1]	4,727	[1]
Commercial Loan [Member] | Investment Grade [Member] | Business Banking and Middle Markets Enterprises [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,098	[1]	6,012	[1]
Commercial Loan [Member] | Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16,472	[1]	16,206	[1]
Commercial Loan [Member] | Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,142	[1]	1,253	[1]
Commercial Loan [Member] | Non-Investment Grade [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16,435		15,952
Commercial Loan [Member] | Non-Investment Grade [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,827		3,730
Commercial Loan [Member] | Non-Investment Grade [Member] | Business Banking and Middle Markets Enterprises [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,627		6,596
Commercial Loan [Member] | Non-Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,246		3,803
Commercial Loan [Member] | Non-Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 1,735		$ 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.