Loans and the Allowance for Credit Losses (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment	$ 1,244	$ 1,743
Allowance for loans collectively evaluated for impairment	50,420	50,956
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	446	341	468	468
Total allowance for credit losses	52,110	53,040	54,863	55,278
Individually evaluated for impairment	6,527	9,532
Collectively evaluated for impairment	3,673,482	3,674,320
Loans acquired with deteriorated credit quality	3,792	3,910
Total Loans	3,683,801	3,687,762
Commercial Real Estate - Land and Construction [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment	850	832
Allowance for loans collectively evaluated for impairment	3,095	2,909
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	32	27	53	74
Total allowance for credit losses	3,977	3,768	4,484	4,916
Individually evaluated for impairment	2,036	2,545
Collectively evaluated for impairment	182,268	189,839
Loans acquired with deteriorated credit quality	498	620
Total Loans	184,802	193,004
Commercial Real Estate - Improved Property [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment	394	911
Allowance for loans collectively evaluated for impairment	22,795	22,703
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	25	25	6	21
Total allowance for credit losses	23,214	23,639	24,905	24,769
Individually evaluated for impairment	4,491	6,987
Collectively evaluated for impairment	1,642,421	1,658,306
Loans acquired with deteriorated credit quality	40	48
Total Loans	1,646,952	1,665,341
Commercial and industrial [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	9,389	9,326
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	304	215	346	323
Total allowance for credit losses	9,693	9,541	11,477	11,737
Individually evaluated for impairment
Collectively evaluated for impairment	495,304	477,594
Loans acquired with deteriorated credit quality	444	431
Total Loans	495,748	478,025
Residential Real Estate [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	6,952	7,182
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	9	6	4	4
Total allowance for credit losses	6,961	7,188	5,713	5,642
Individually evaluated for impairment
Collectively evaluated for impairment	806,310	791,475
Loans acquired with deteriorated credit quality	2,218	2,227
Total Loans	808,528	793,702
Home equity [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	2,475	2,458
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	57	49	44	33
Total allowance for credit losses	2,532	2,507	2,316	1,995
Individually evaluated for impairment
Collectively evaluated for impairment	278,721	277,126
Loans acquired with deteriorated credit quality	91	100
Total Loans	278,812	277,226
Consumer [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	5,161	5,557
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments	19	19	15	13
Total allowance for credit losses	5,180	5,576	5,429	5,423
Individually evaluated for impairment
Collectively evaluated for impairment	268,458	279,980
Loans acquired with deteriorated credit quality	501	484
Total Loans	268,959	280,464
Overdraft [Member]
Allowance for credit losses and recorded investments in loans
Allowance for loans individually evaluated for impairment
Allowance for loans collectively evaluated for impairment	553	821
Allowance for loans acquired with deteriorated credit quality
Allowance for loan commitments
Total allowance for credit losses	553	821	539	796
Individually evaluated for impairment
Collectively evaluated for impairment
Loans acquired with deteriorated credit quality
Total Loans