Asset Quality - Financing Receivable Credit Quality Indicators (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 57,953	$ 57,381	$ 55,445
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	28,783	27,982	26,224
Commercial, Financial and Agricultural [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	286		500
Commercial, Financial and Agricultural [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,267		1,010
Commercial, Financial and Agricultural [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	25,684		23,361
Commercial, Financial and Agricultural [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	648		551
Commercial, Financial and Agricultural [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	898		802
RE Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	8,162		7,876
RE Commercial [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	6		1
RE Commercial [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2		57
RE Commercial [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	7,604		7,267
RE Commercial [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	325		337
RE Commercial [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	225		214
Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,142		1,007
Commercial Real Estate Construction Financing Receivable [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1		1
Commercial Real Estate Construction Financing Receivable [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans			1
Commercial Real Estate Construction Financing Receivable [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	992		826
Commercial Real Estate Construction Financing Receivable [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	127		84
Commercial Real Estate Construction Financing Receivable [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	22		95
Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,064		4,396
Commercial Leases [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	535		805
Commercial Leases [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	516		448
Commercial Leases [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,842		2,964
Commercial Leases [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	104		101
Commercial Leases [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	67		78
Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	42,151		39,503
Commercial Loan Portfolios [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	828		1,307
Commercial Loan Portfolios [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,785		1,516
Commercial Loan Portfolios [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	37,122		34,418
Commercial Loan Portfolios [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,204		1,073
Commercial Loan Portfolios [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 1,212		$ 1,189