Loans and the Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	$ 44,654	$ 47,368	$ 47,368
Allowance for loan commitments, beginning balance	455	602	602
Total beginning allowance for credit losses	45,109	47,970	47,970
Provision for loan losses	1,266	2,256
Provision for loan commitments	23	(57)
Total provision for credit losses	1,289	2,199	2,199
Charge-offs	(2,517)	(4,735)
Recoveries	770	594
Net charge-offs	(1,747)	(4,141)
Allowance for loan losses, ending balance	44,173	45,483	44,654
Allowance for loan commitments, ending balance	478	545	455
Total ending allowance for credit losses	44,651	46,028	45,109
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	5,654	6,056	6,056
Allowance for loan commitments, beginning balance	194	301	301
Total beginning allowance for credit losses	5,848	6,357	6,357
Provision for loan losses	(323)	(1,051)
Provision for loan commitments	(16)	(8)
Total provision for credit losses	(339)	(1,059)
Allowance for loan losses, ending balance	5,331	5,005
Allowance for loan commitments, ending balance	178	293
Total ending allowance for credit losses	5,509	5,298
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	17,573	18,157	18,157
Allowance for loan commitments, beginning balance	10	62	62
Total beginning allowance for credit losses	17,583	18,219	18,219
Provision for loan losses	903	(509)
Provision for loan commitments	8	(53)
Total provision for credit losses	911	(562)
Charge-offs	(577)	(493)
Recoveries	136	120
Net charge-offs	(441)	(373)
Allowance for loan losses, ending balance	18,035	17,275
Allowance for loan commitments, ending balance	18	9
Total ending allowance for credit losses	18,053	17,284
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	9,063	9,925	9,925
Allowance for loan commitments, beginning balance	112	130	130
Total beginning allowance for credit losses	9,175	10,055	10,055
Provision for loan losses	(44)	2,128
Provision for loan commitments	8	3
Total provision for credit losses	(36)	2,131
Charge-offs	(122)	(2,276)
Recoveries	114	65
Net charge-offs	(8)	(2,211)
Allowance for loan losses, ending balance	9,011	9,842
Allowance for loan commitments, ending balance	120	133
Total ending allowance for credit losses	9,131	9,975
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	5,382	5,673	5,673
Allowance for loan commitments, beginning balance	9	5	5
Total beginning allowance for credit losses	5,391	5,678	5,678
Provision for loan losses	(208)	869
Provision for loan commitments	4
Total provision for credit losses	(204)	869
Charge-offs	(358)	(879)
Recoveries	218	109
Net charge-offs	(140)	(770)
Allowance for loan losses, ending balance	5,034	5,772
Allowance for loan commitments, ending balance	13	5
Total ending allowance for credit losses	5,047	5,777
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	2,329	2,017	2,017
Allowance for loan commitments, beginning balance	90	85	85
Total beginning allowance for credit losses	2,419	2,102	2,102
Provision for loan losses	747	153
Provision for loan commitments	17	1
Total provision for credit losses	764	154
Charge-offs	(589)	(155)
Recoveries	10	11
Net charge-offs	(579)	(144)
Allowance for loan losses, ending balance	2,497	2,026
Allowance for loan commitments, ending balance	107	86
Total ending allowance for credit losses	2,604	2,112
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	4,078	5,020	5,020
Allowance for loan commitments, beginning balance	40	19	19
Total beginning allowance for credit losses	4,118	5,039	5,039
Provision for loan losses	133	305
Provision for loan commitments	2
Total provision for credit losses	135	305
Charge-offs	(717)	(752)
Recoveries	229	227
Net charge-offs	(488)	(525)
Allowance for loan losses, ending balance	3,723	4,800
Allowance for loan commitments, ending balance	42	19
Total ending allowance for credit losses	3,765	4,819
Deposit Overdraft [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, beginning balance	575	520	520
Total beginning allowance for credit losses	575	520	520
Provision for loan losses	58	361
Total provision for credit losses	58	361
Charge-offs	(154)	(180)
Recoveries	63	62
Net charge-offs	(91)	(118)
Allowance for loan losses, ending balance	542	763
Total ending allowance for credit losses	$ 542	$ 763