Asset Quality - Financing Receivable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 86,038	$ 59,876	$ 57,381
Commercial Loan Portfolios [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	39,656	31,240
Commercial Loan Portfolios [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	14,789	7,959
Commercial Loan Portfolios [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,319	1,053
Commercial Loan Portfolios [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,685	4,020
Commercial Loan Portfolios [Member] | Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	61,449	44,272
Commercial Loan Portfolios [Member] | Pass [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	37,845	29,921
Commercial Loan Portfolios [Member] | Pass [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	14,308	7,800
Commercial Loan Portfolios [Member] | Pass [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,287	1,007
Commercial Loan Portfolios [Member] | Pass [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,632	3,967
Commercial Loan Portfolios [Member] | Pass [Member] | Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	59,072	42,695
Commercial Loan Portfolios [Member] | Criticized Accruing [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,514	1,236
Commercial Loan Portfolios [Member] | Criticized Accruing [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	455	139
Commercial Loan Portfolios [Member] | Criticized Accruing [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	30	37
Commercial Loan Portfolios [Member] | Criticized Accruing [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	45	38
Commercial Loan Portfolios [Member] | Criticized Accruing [Member] | Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,044	1,450
Commercial Loan Portfolios [Member] | Criticized Nonaccruing [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	297	83
Commercial Loan Portfolios [Member] | Criticized Nonaccruing [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	26	20
Commercial Loan Portfolios [Member] | Criticized Nonaccruing [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2	9
Commercial Loan Portfolios [Member] | Criticized Nonaccruing [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8	15
Commercial Loan Portfolios [Member] | Criticized Nonaccruing [Member] | Commercial Loan Portfolios [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	333	127
Commercial Portfolio Segment, PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	460	0
Commercial Portfolio Segment, PCI [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	112	0
Commercial Portfolio Segment, PCI [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	322	0
Commercial Portfolio Segment, PCI [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	26	0
Commercial Portfolio Segment, PCI [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial Portfolio Segment, PCI [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	172	0
Commercial Portfolio Segment, PCI [Member] | Pass [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12	0
Commercial Portfolio Segment, PCI [Member] | Pass [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	139	0
Commercial Portfolio Segment, PCI [Member] | Pass [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	21	0
Commercial Portfolio Segment, PCI [Member] | Pass [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial Portfolio Segment, PCI [Member] | Criticized Accruing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	288	0
Commercial Portfolio Segment, PCI [Member] | Criticized Accruing [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	100	0
Commercial Portfolio Segment, PCI [Member] | Criticized Accruing [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	183	0
Commercial Portfolio Segment, PCI [Member] | Criticized Accruing [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5	0
Commercial Portfolio Segment, PCI [Member] | Criticized Accruing [Member] | Commercial Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0