Asset Quality - Financing Receivable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 60,085	$ 57,381	$ 56,155
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	31,095	$ 27,982	26,683
Commercial, Financial and Agricultural [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	384		342
Commercial, Financial and Agricultural [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,439		1,147
Commercial, Financial and Agricultural [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	27,438		23,822
Commercial, Financial and Agricultural [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	639		594
Commercial, Financial and Agricultural [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,195		778
RE Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	8,180		8,276
RE Commercial [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	3		2
RE Commercial [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	4		2
RE Commercial [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	7,690		7,736
RE Commercial [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	272		298
RE Commercial [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	211		238
Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,070		1,036
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	935		895
Commercial Real Estate Construction Financing Receivable [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	89		100
Commercial Real Estate Construction Financing Receivable [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	46		40
Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	3,929		4,135
Commercial Leases [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	501		528
Commercial Leases [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	478		596
Commercial Leases [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	2,808		2,848
Commercial Leases [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	88		75
Commercial Leases [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	54		88
Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	44,274		40,130
Commercial Loan Portfolios [Member] | AAA-AA [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	888		873
Commercial Loan Portfolios [Member] | A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,921		1,745
Commercial Loan Portfolios [Member] | BBB-BB [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	38,871		35,301
Commercial Loan Portfolios [Member] | B [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	1,088		1,067
Commercial Loan Portfolios [Member] | CCC-C [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 1,506		$ 1,144