Allowance for Loan Losses Allowance by impairment analysis method (Details) - USD ($)		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		$ 1,167,000	$ 1,629,000
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		75,260,000	76,871,000
Loans and Leases Receivable, Allowance		76,427,000	78,500,000	$ 77,263,000	$ 75,838,000
Financing Receivable, Individually Evaluated for Impairment		36,942,000	39,172,000
Financing Receivable, Collectively Evaluated for Impairment		5,621,239,000	5,680,040,000
Total loans		5,658,181,000	5,719,212,000
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		0	0
Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		23,000	270,000
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		14,320,000	15,544,000
Loans and Leases Receivable, Allowance		14,343,000	15,814,000	14,337,000	14,114,000
Financing Receivable, Individually Evaluated for Impairment		2,494,000	2,274,000
Financing Receivable, Collectively Evaluated for Impairment		1,067,477,000	1,109,281,000
Total loans		1,069,971,000	1,111,555,000
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		671,000	713,000
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		43,848,000	43,502,000
Loans and Leases Receivable, Allowance		44,519,000	44,215,000	43,552,000	43,854,000
Financing Receivable, Individually Evaluated for Impairment		19,199,000	19,462,000
Financing Receivable, Collectively Evaluated for Impairment		1,906,320,000	1,894,672,000
Total loans		1,925,519,000	1,914,134,000
Construction and land [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		0	172,000
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		5,551,000	6,150,000
Loans and Leases Receivable, Allowance		5,551,000	6,322,000	5,411,000	4,041,000
Financing Receivable, Individually Evaluated for Impairment		2,850,000	3,297,000
Financing Receivable, Collectively Evaluated for Impairment		163,824,000	180,137,000
Total loans		166,674,000	183,434,000
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		473,000	474,000
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		10,161,000	10,070,000
Loans and Leases Receivable, Allowance		10,634,000	10,544,000	10,397,000	10,374,000
Financing Receivable, Individually Evaluated for Impairment		12,399,000	14,139,000
Financing Receivable, Collectively Evaluated for Impairment		2,204,476,000	2,215,401,000
Total loans		2,216,875,000	2,229,540,000
Home equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		1,079,000	1,085,000
Loans and Leases Receivable, Allowance		1,079,000	1,085,000	1,016,000	1,003,000
Financing Receivable, Individually Evaluated for Impairment		0	0
Financing Receivable, Collectively Evaluated for Impairment		118,807,000	119,828,000
Total loans		118,807,000	119,828,000
Consumer and other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment		0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment		301,000	520,000
Loans and Leases Receivable, Allowance		301,000	520,000	508,000	382,000
Financing Receivable, Individually Evaluated for Impairment		0	0
Financing Receivable, Collectively Evaluated for Impairment		160,335,000	160,721,000
Total loans		160,335,000	160,721,000
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance	[1]	$ 0	$ 0	$ 2,042,000	$ 2,070,000

[1] As of December 31, 2015, the unallocated reserve was allocated to the qualitative factors as part of the general reserves (ASC 450).