Loans and the Allowance for Credit Losses - Allowance for Credit Losses and Recorded Investment in Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	$ 1,068	$ 732
Allowance for loans collectively evaluated for impairment	44,673	46,636
Allowance for loan commitments	536	602	317	341
Total allowance for credit losses	46,277	47,970	50,698	53,040
Individually evaluated for impairment	7,348	6,075
Collectively evaluated for impairment	3,938,770	3,888,842
Total Loans	3,946,118	3,894,917
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	5,651	6,056
Allowance for loan commitments	281	301	38	27
Total allowance for credit losses	5,932	6,357	3,975	3,768
Collectively evaluated for impairment	255,499	263,117
Total Loans	255,499	263,117
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	39	51
Allowance for loans collectively evaluated for impairment	17,269	18,106
Allowance for loan commitments	10	62	23	25
Total allowance for credit losses	17,318	18,219	24,517	23,639
Individually evaluated for impairment	4,264	4,321
Collectively evaluated for impairment	1,681,109	1,645,481
Total Loans	1,685,373	1,649,802
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	1,029	681
Allowance for loans collectively evaluated for impairment	8,925	9,244
Allowance for loan commitments	131	130	138	215
Total allowance for credit losses	10,085	10,055	8,610	9,541
Individually evaluated for impairment	3,084	1,754
Collectively evaluated for impairment	575,581	554,495
Total Loans	578,665	556,249
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	5,289	5,673
Allowance for loan commitments	5	5	6	6
Total allowance for credit losses	5,294	5,678	5,815	7,188
Collectively evaluated for impairment	898,357	890,804
Total Loans	898,357	890,804
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	2,132	2,017
Allowance for loan commitments	90	85	93	49
Total allowance for credit losses	2,222	2,102	2,216	2,507
Collectively evaluated for impairment	295,127	284,687
Total Loans	295,127	284,687
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	4,564	5,020
Allowance for loan commitments	19	19	19	19
Total allowance for credit losses	4,583	5,039	4,976	5,576
Collectively evaluated for impairment	233,097	250,258
Total Loans	233,097	250,258
Deposit Overdraft [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	843	520
Total allowance for credit losses	$ 843	$ 520	$ 589	$ 821