Asset Quality (Details 5) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Mar. 31, 2011
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	$ 49,226	$ 48,552
Commercial, financial and agricultural [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	19,787	16,440
RE-Commercial [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	7,807	8,806
RE-Construction [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	1,273	1,845
Commercial Lease [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	5,755	6,207
Commercial Loan [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	34,622	33,298
AAA-AA [Member] | Commercial, financial and agricultural [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	165	95
AAA-AA [Member] | RE-Commercial [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	3	2
AAA-AA [Member] | RE-Construction [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	3
AAA-AA [Member] | Commercial Lease [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	599	645
AAA-AA [Member] | Commercial Loan [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	770	742
A [Member] | Commercial, financial and agricultural [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	785	712
A [Member] | RE-Commercial [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	62	84
A [Member] | RE-Construction [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	1	5
A [Member] | Commercial Lease [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	1,156	1,246
A [Member] | Commercial Loan [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	2,004	2,047
BBB-BB [Member] | Commercial, financial and agricultural [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	16,801	12,646
BBB-BB [Member] | RE-Commercial [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	6,007	6,045
BBB-BB [Member] | RE-Construction [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	788	801
BBB-BB [Member] | Commercial Lease [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	3,623	3,655
BBB-BB [Member] | Commercial Loan [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	27,219	23,147
B [Member] | Commercial, financial and agricultural [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	848	1,125
B [Member] | RE-Commercial [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	568	954
B [Member] | RE-Construction [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	165	309
B [Member] | Commercial Lease [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	236	365
B [Member] | Commercial Loan [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	1,817	2,753
CCC-C [Member] | Commercial, financial and agricultural [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	1,188	1,862
CCC-C [Member] | RE-Commercial [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	1,167	1,721
CCC-C [Member] | RE-Construction [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	316	730
CCC-C [Member] | Commercial Lease [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	141	296
CCC-C [Member] | Commercial Loan [Member]
Commercial Credit Exposure Credit Risk Profile by Creditworthiness Category
Total Loans	$ 2,812	$ 4,609