Loans and the Allowance for Credit Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	$ 3,443	$ 1,743
Allowance for loans collectively evaluated for impairment	46,938	50,956
Allowance for loan commitments	317	341	351	468
Total allowance for credit losses	50,698	53,040	53,961	55,278
Individually evaluated for impairment	9,201	9,532
Collectively evaluated for impairment	3,792,181	3,678,230
Total Loans	3,801,382	3,687,762
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	839	832
Allowance for loans collectively evaluated for impairment	3,098	2,909
Allowance for loan commitments	38	27	49	74
Total allowance for credit losses	3,975	3,768	4,524	4,916
Individually evaluated for impairment	2,025	2,545
Collectively evaluated for impairment	193,473	190,459
Total Loans	195,498	193,004
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	2,604	911
Allowance for loans collectively evaluated for impairment	21,890	22,703
Allowance for loan commitments	23	25	10	21
Total allowance for credit losses	24,517	23,639	24,731	24,769
Individually evaluated for impairment	7,176	6,987
Collectively evaluated for impairment	1,678,409	1,658,354
Total Loans	1,685,585	1,665,341
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	8,472	9,326
Allowance for loan commitments	138	215	224	323
Total allowance for credit losses	8,610	9,541	10,292	11,737
Individually evaluated for impairment
Collectively evaluated for impairment	542,071	478,025
Total Loans	542,071	478,025
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	5,809	7,182
Allowance for loan commitments	6	6	5	4
Total allowance for credit losses	5,815	7,188	5,785	5,642
Individually evaluated for impairment
Collectively evaluated for impairment	831,362	793,702
Total Loans	831,362	793,702
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	2,123	2,458
Allowance for loan commitments	93	49	48	33
Total allowance for credit losses	2,216	2,507	2,489	1,995
Individually evaluated for impairment
Collectively evaluated for impairment	280,368	277,226
Total Loans	280,368	277,226
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	4,957	5,557
Allowance for loan commitments	19	19	15	13
Total allowance for credit losses	4,976	5,576	5,434	5,423
Individually evaluated for impairment
Collectively evaluated for impairment	266,498	280,464
Total Loans	266,498	280,464
Deposit Overdraft [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	589	821
Allowance for loan commitments
Total allowance for credit losses	589	821	706	796
Individually evaluated for impairment
Collectively evaluated for impairment
Total Loans