Asset Quality - Financing Receivable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 59,876	$ 57,381	$ 54,457
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	31,240	27,982
Commercial, Financial and Agricultural [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	384	311
Commercial, Financial and Agricultural [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,475	1,272
Commercial, Financial and Agricultural [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	27,321	24,949
Commercial, Financial and Agricultural [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	740	686
Commercial, Financial and Agricultural [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,320	764
RE Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	7,959	8,047
RE Commercial [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	7	2
RE Commercial [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2	1
RE Commercial [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	7,492	7,527
RE Commercial [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	301	287
RE Commercial [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	157	230
Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,053	1,100
Commercial Real Estate Construction Financing Receivable [Member] | AAA-AA [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	920	956
Commercial Real Estate Construction Financing Receivable [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	86	105
Commercial Real Estate Construction Financing Receivable [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	47	38
Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4,020	4,252
Commercial Leases [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	503	513
Commercial Leases [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	509	608
Commercial Leases [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,858	2,952
Commercial Leases [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	99	112
Commercial Leases [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	51	67
Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	44,272	41,381
Commercial Loan Portfolios [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	894	827
Commercial Loan Portfolios [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,986	1,881
Commercial Loan Portfolios [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	38,591	36,384
Commercial Loan Portfolios [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,226	1,190
Commercial Loan Portfolios [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 1,575	$ 1,099