Allowance for Loan Losses Allowance by impairment analysis method (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Acquired with Deteriorated Credit Quality	$ 0	$ 0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	73,159,000	71,653,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	4,104,000	4,185,000
Loans and Leases Receivable, Allowance	77,263,000	75,838,000	76,605,000	76,371,000
Financing Receivable, Collectively Evaluated for Impairment	5,244,571,000	5,207,593,000
Financing Receivable, Individually Evaluated for Impairment	50,442,000	62,343,000
Total loans	5,295,013,000	5,269,936,000
Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	14,259,000	14,023,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	78,000	91,000
Loans and Leases Receivable, Allowance	14,337,000	14,114,000	12,200,000	12,837,000
Financing Receivable, Collectively Evaluated for Impairment	965,706,000	950,183,000
Financing Receivable, Individually Evaluated for Impairment	1,132,000	2,902,000
Total loans	966,838,000	953,085,000
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	41,009,000	41,262,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	2,543,000	2,592,000
Loans and Leases Receivable, Allowance	43,552,000	43,854,000	46,366,000	44,979,000
Financing Receivable, Collectively Evaluated for Impairment	1,734,987,000	1,757,839,000
Financing Receivable, Individually Evaluated for Impairment	28,054,000	30,564,000
Total loans	1,763,041,000	1,788,403,000
Construction and land [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	5,239,000	3,869,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	172,000	172,000
Loans and Leases Receivable, Allowance	5,411,000	4,041,000	4,972,000	4,465,000
Financing Receivable, Collectively Evaluated for Impairment	133,622,000	113,928,000
Financing Receivable, Individually Evaluated for Impairment	3,472,000	11,421,000
Total loans	137,094,000	125,349,000
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	9,086,000	9,044,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,311,000	1,330,000
Loans and Leases Receivable, Allowance	10,397,000	10,374,000	9,746,000	10,732,000
Financing Receivable, Collectively Evaluated for Impairment	2,126,309,000	2,115,696,000
Financing Receivable, Individually Evaluated for Impairment	16,727,000	16,399,000
Total loans	2,143,036,000	2,132,095,000
Home equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,016,000	1,003,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Loans and Leases Receivable, Allowance	1,016,000	1,003,000	963,000	1,020,000
Financing Receivable, Collectively Evaluated for Impairment	116,140,000	114,809,000
Financing Receivable, Individually Evaluated for Impairment	50,000	50,000
Total loans	116,190,000	114,859,000
Consumer and other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	508,000	382,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Loans and Leases Receivable, Allowance	508,000	382,000	330,000	322,000
Financing Receivable, Collectively Evaluated for Impairment	167,807,000	155,138,000
Financing Receivable, Individually Evaluated for Impairment	1,007,000	1,007,000
Total loans	168,814,000	156,145,000
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	2,042,000	2,070,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Loans and Leases Receivable, Allowance	2,042,000	2,070,000	2,028,000	2,016,000
Financing Receivable, Collectively Evaluated for Impairment	0	0
Financing Receivable, Individually Evaluated for Impairment	0	0
Total loans	$ 0	$ 0