Loans and Allowance for Loan Losses - Information about Impaired Loans by Class (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Impaired loans	$ 648	$ 626
Allowance for impaired loans	47	42
Commercial and industrial [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	266	220
Allowance for impaired loans	30	24
Commercial mortgage [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	57	75
Allowance for impaired loans	3
Construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	2	35
Commercial portfolio [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	325	330
Allowance for impaired loans	33	24
Residential mortgage [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	297	271
Allowance for impaired loans	14	18
Home equity and other consumer loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	22	21
Consumer portfolio [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	319	292
Allowance for impaired loans	14	18
Loans excluding purchased credit-impaired loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	644	622
Allowance for impaired loans	47	42
Purchased Credit Impaired [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	4	4
With an Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	454	360
Unpaid principal balance	475	388
With an Allowance [Member] | Commercial and industrial [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	230	156
Unpaid principal balance	234	167
With an Allowance [Member] | Commercial mortgage [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	29	15
Unpaid principal balance	31	18
With an Allowance [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	2
Unpaid principal balance	3
With an Allowance [Member] | Commercial portfolio [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	261	171
Unpaid principal balance	268	185
With an Allowance [Member] | Residential mortgage [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	189	186
Unpaid principal balance	203	199
With an Allowance [Member] | Home equity and other consumer loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	3	3
Unpaid principal balance	3	3
With an Allowance [Member] | Consumer portfolio [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	192	189
Unpaid principal balance	206	202
With an Allowance [Member] | Loans excluding purchased credit-impaired loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	453	360
Unpaid principal balance	474	387
With an Allowance [Member] | Purchased Credit Impaired [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	1
Unpaid principal balance	1	1
Without Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	194	266
Unpaid principal balance	240	314
Without Allowance [Member] | Commercial and industrial [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	36	64
Unpaid principal balance	37	64
Without Allowance [Member] | Commercial mortgage [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	28	60
Unpaid principal balance	38	70
Without Allowance [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans		35
Unpaid principal balance		38
Without Allowance [Member] | Commercial portfolio [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	64	159
Unpaid principal balance	75	172
Without Allowance [Member] | Residential mortgage [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	108	85
Unpaid principal balance	123	97
Without Allowance [Member] | Home equity and other consumer loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	19	18
Unpaid principal balance	33	31
Without Allowance [Member] | Consumer portfolio [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	127	103
Unpaid principal balance	156	128
Without Allowance [Member] | Loans excluding purchased credit-impaired loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	191	262
Unpaid principal balance	231	300
Without Allowance [Member] | Purchased Credit Impaired [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans	3	4
Unpaid principal balance	$ 9	$ 14