Allowance for Loan Losses Allowance by impairment analysis method (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 344	$ 654
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	72,554	74,088
Loans and Leases Receivable, Allowance	72,898	74,742	$ 78,031	$ 78,077
Financing Receivable, Individually Evaluated for Impairment	24,014	22,577
Financing Receivable, Collectively Evaluated for Impairment	6,578,313	6,482,451
Total loans	6,602,327	6,505,028
Commercial and Industrial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	58
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	11,443	11,677
Loans and Leases Receivable, Allowance	11,443	11,735	12,291	12,751
Financing Receivable, Individually Evaluated for Impairment	2,100	1,676
Financing Receivable, Collectively Evaluated for Impairment	949,750	938,014
Total loans	951,850	939,690
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	241	362
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	45,875	46,458
Loans and Leases Receivable, Allowance	46,116	46,820	51,164	50,412
Financing Receivable, Individually Evaluated for Impairment	8,472	8,687
Financing Receivable, Collectively Evaluated for Impairment	2,456,531	2,431,533
Total loans	2,465,003	2,440,220
Construction and land [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	4,533	4,949
Loans and Leases Receivable, Allowance	4,533	4,949	3,197	3,039
Financing Receivable, Individually Evaluated for Impairment	109	109
Financing Receivable, Collectively Evaluated for Impairment	165,131	164,881
Total loans	165,240	164,990
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	83	89
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	9,813	9,684
Loans and Leases Receivable, Allowance	9,896	9,773	10,090	10,449
Financing Receivable, Individually Evaluated for Impairment	11,538	10,165
Financing Receivable, Collectively Evaluated for Impairment	2,725,831	2,672,368
Total loans	2,737,369	2,682,533
Home equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	20	20
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	764	815
Loans and Leases Receivable, Allowance	784	835	987	1,035
Financing Receivable, Individually Evaluated for Impairment	1,795	1,815
Financing Receivable, Collectively Evaluated for Impairment	92,536	98,143
Total loans	94,331	99,958
Consumer and other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	125
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	126	505
Loans and Leases Receivable, Allowance	126	630	$ 302	$ 391
Financing Receivable, Individually Evaluated for Impairment	0	125
Financing Receivable, Collectively Evaluated for Impairment	188,534	177,512
Total loans	$ 188,534	$ 177,637