Loans and leases and the allowance for credit losses - Loan Modification Activities that were Considered Troubled Debt Restructurings (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 Modification	Mar. 31, 2014 Modification
Financing Receivable, Modifications [Line Items]
Number of modifications	150	243
Pre-modification, Recorded Investment	$ 20,132	$ 32,530
Post-modification, Recorded investment	18,002	32,500
Financial effects of modification, Recorded investment	(2,130)	(30)
Financial Effects of Modification, Interest	(157)	(869)
Principal Deferral [Member] | Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	21	30
Pre-modification, Recorded Investment	1,572	14,954
Post-modification, Recorded investment	1,557	14,848
Financial effects of modification, Recorded investment	(15)	(106)
Principal Deferral [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	7	13
Pre-modification, Recorded Investment	3,792	7,044
Post-modification, Recorded investment	3,776	7,002
Financial effects of modification, Recorded investment	(16)	(42)
Principal Deferral [Member] | Other Commercial Construction [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications		1
Pre-modification, Recorded Investment		151
Post-modification, Recorded investment		151
Principal Deferral [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	7	13
Pre-modification, Recorded Investment	721	1,602
Post-modification, Recorded investment	742	1,663
Financial effects of modification, Recorded investment	21	61
Principal Deferral [Member] | Residential Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications		2
Pre-modification, Recorded Investment		166
Post-modification, Recorded investment		202
Financial effects of modification, Recorded investment		36
Principal Deferral [Member] | Home Equity Lines and Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	1	3
Pre-modification, Recorded Investment	21	280
Post-modification, Recorded investment	21	280
Principal Deferral [Member] | Automobile [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	35	80
Pre-modification, Recorded Investment	303	993
Post-modification, Recorded investment	303	993
Principal Deferral [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	22	8
Pre-modification, Recorded Investment	296	55
Post-modification, Recorded investment	296	55
Principal Deferral [Member] | Residential Builder and Developer [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	1
Pre-modification, Recorded Investment	1,398
Post-modification, Recorded investment	1,398
Interest Rate Reduction [Member] | Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	1
Pre-modification, Recorded Investment	99
Post-modification, Recorded investment	99
Financial Effects of Modification, Interest	(19)
Interest Rate Reduction [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications		1
Pre-modification, Recorded Investment		98
Post-modification, Recorded investment		104
Financial effects of modification, Recorded investment		6
Financial Effects of Modification, Interest		(32)
Interest Rate Reduction [Member] | Automobile [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	3
Pre-modification, Recorded Investment	42
Post-modification, Recorded investment	42
Financial Effects of Modification, Interest	(3)
Other [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications		1
Pre-modification, Recorded Investment		188
Post-modification, Recorded investment		188
Other [Member] | Automobile [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	10	11
Pre-modification, Recorded Investment	20	61
Post-modification, Recorded investment	20	61
Other [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	5	1
Pre-modification, Recorded Investment	59	45
Post-modification, Recorded investment	59	45
Combination of Concession Types [Member] | Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	3	2
Pre-modification, Recorded Investment	9,155	41
Post-modification, Recorded investment	6,989	39
Financial effects of modification, Recorded investment	(2,166)	(2)
Financial Effects of Modification, Interest		(4)
Combination of Concession Types [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	4	1
Pre-modification, Recorded Investment	1,646	346
Post-modification, Recorded investment	1,637	401
Financial effects of modification, Recorded investment	(9)	55
Financial Effects of Modification, Interest	(52)	(104)
Combination of Concession Types [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	3	14
Pre-modification, Recorded Investment	294	2,188
Post-modification, Recorded investment	349	2,160
Financial effects of modification, Recorded investment	55	(28)
Financial Effects of Modification, Interest	(34)	(282)
Combination of Concession Types [Member] | Residential Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	1	10
Pre-modification, Recorded Investment	210	1,746
Post-modification, Recorded investment	210	1,736
Financial effects of modification, Recorded investment		(10)
Financial Effects of Modification, Interest	(4)	(61)
Combination of Concession Types [Member] | Home Equity Lines and Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	5	15
Pre-modification, Recorded Investment	196	1,856
Post-modification, Recorded investment	196	1,856
Financial Effects of Modification, Interest	(13)	(172)
Combination of Concession Types [Member] | Automobile [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	8	23
Pre-modification, Recorded Investment	84	250
Post-modification, Recorded investment	84	250
Financial Effects of Modification, Interest	(7)	(26)
Combination of Concession Types [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications	13	14
Pre-modification, Recorded Investment	224	466
Post-modification, Recorded investment	224	466
Financial Effects of Modification, Interest	$ (25)	$ (188)