Asset Quality - Financing Receivable Credit Quality Indicators (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Commercial, Financial and Agricultural [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 292	$ 166
Commercial, Financial and Agricultural [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	774	755
Commercial, Financial and Agricultural [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	21,837	19,229
Commercial, Financial and Agricultural [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	487	940
Commercial, Financial and Agricultural [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	927	888
Commercial, Financial and Agricultural [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	24,317	21,978
RE Commercial [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans		1
RE Commercial [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	73	63
RE Commercial [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	6,867	6,137
RE Commercial [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	294	585
RE Commercial [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	308	740
RE Commercial [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	7,542	7,526
RE Construction [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1	1
RE Construction [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1	1
RE Construction [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	879	759
RE Construction [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	26	38
RE Construction [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	151	268
RE Construction [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,058	1,067
Commercial Lease Financing [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	454	465
Commercial Lease Financing [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	866	1,107
Commercial Lease Financing [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	3,021	3,087
Commercial Lease Financing [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	133	188
Commercial Lease Financing [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	76	113
Commercial Lease Financing [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,550	4,960
Commercial Loans [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	747	633
Commercial Loans [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,714	1,926
Commercial Loans [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	32,604	29,212
Commercial Loans [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	940	1,751
Commercial Loans [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,462	2,009
Commercial Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 37,467	$ 35,531