Loans - Summary of Past Due Status of Loans (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	$ 1,989	$ 2,273
Days Past Due, 30-89 days	663	813
Days Past Due, 90 + days	1,231	1,356
Total Past Due	3,883	4,442
Current	57,569	47,425
Total	61,452	51,867	49,600
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	1,469	1,935
Days Past Due, 30-89 days	163	207
Days Past Due, 90 + days	191	397
Total Past Due	1,823	2,539
Current	40,745	31,110
Total	42,568	33,649
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	94	72
Days Past Due, 30-89 days	72	31
Days Past Due, 90 + days	105	231
Total Past Due	271	334
Current	7,649	7,526
Total	7,920	7,860	7,773
Commercial Loans [Member] | Business Banking and Middle Markets Enterprises [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	486	615
Days Past Due, 30-89 days	82	58
Days Past Due, 90 + days	52	71
Total Past Due	620	744
Current	11,877	9,481
Total	12,497	10,225	9,997
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	284	898
Days Past Due, 30-89 days		34
Days Past Due, 90 + days	8	74
Total Past Due	292	1,006
Current	18,719	11,652
Total	19,011	12,658	12,055
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	605	350
Days Past Due, 30-89 days	9	84
Days Past Due, 90 + days	26	21
Total Past Due	640	455
Current	2,500	2,451
Total	3,140	2,906	1,794
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	520	338
Days Past Due, 30-89 days	500	606
Days Past Due, 90 + days	1,040	959
Total Past Due	2,060	1,903
Current	16,824	16,315
Total	18,884	18,218
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	380	181
Days Past Due, 30-89 days	20	54
Days Past Due, 90 + days	86	89
Total Past Due	486	324
Current	1,901	2,239
Total	2,387	2,563	2,618
Consumer Loans [Member] | Other Residential Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	104	109
Days Past Due, 30-89 days	460	526
Days Past Due, 90 + days	907	815
Total Past Due	1,471	1,450
Current	13,615	12,663
Total	15,086	14,113
Consumer Loans [Member] | Credit Card Receivables [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	29	37
Days Past Due, 30-89 days	15	20
Days Past Due, 90 + days	15	20
Total Past Due	59	77
Current	737	751
Total	796	828	788
Consumer Loans [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due, 1-29 days	7	11
Days Past Due, 30-89 days	5	6
Days Past Due, 90 + days	32	35
Total Past Due	44	52
Current	571	662
Total	$ 615	$ 714	$ 733