Loans and Allowance for Loan Losses - Loans in Commercial Portfolio Segment and Commercial Loans within Purchased Credit-Impaired Loans Segment Monitored for Credit Quality (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 37,635	$ 32,621
Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,139	21,779
Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	808	627
Commercial mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,821	9,740
Commercial portfolio [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	36,768	32,146
Purchased Credit Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	867	475
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	35,480	30,890
Pass [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	22,248	20,961
Pass [Member] | Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	792	610
Pass [Member] | Commercial mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,366	9,298
Pass [Member] | Commercial portfolio [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	35,406	30,869
Pass [Member] | Purchased Credit Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	74	21
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,145	802
Special Mention [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	535	438
Special Mention [Member] | Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16	17
Special Mention [Member] | Commercial mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	243	194
Special Mention [Member] | Commercial portfolio [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	794	649
Special Mention [Member] | Purchased Credit Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	351	153
Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,010	929
Classified [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	356	380
Classified [Member] | Commercial mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	212	248
Classified [Member] | Commercial portfolio [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	568	628
Classified [Member] | Purchased Credit Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 442	$ 301