Allowance for Loan Losses Allowance by impairment analysis method (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	$ 23,716		$ 22,577
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	1,000		654
Financing Receivable, Collectively Evaluated for Impairment	6,743,407		6,482,451
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	72,464		74,088
Loans and Leases Receivable, Net of Deferred Income	6,767,123		6,505,028
Loans and Leases Receivable, Allowance	73,464	$ 72,898	74,742	$ 75,009	$ 78,031	$ 78,077
Commercial and Industrial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	2,318		1,676
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	134		58
Financing Receivable, Collectively Evaluated for Impairment	1,019,757		938,014
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	12,247		11,677
Loans and Leases Receivable, Net of Deferred Income	1,022,075		939,690
Loans and Leases Receivable, Allowance	12,381	11,443	11,735	11,672	12,291	12,751
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	8,358		8,687
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	187		362
Financing Receivable, Collectively Evaluated for Impairment	2,496,163		2,431,533
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	44,996		46,458
Loans and Leases Receivable, Net of Deferred Income	2,504,521		2,440,220
Loans and Leases Receivable, Allowance	45,183	46,116	46,820	48,136	51,164	50,412
Construction and land [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	0		109
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Financing Receivable, Collectively Evaluated for Impairment	172,024		164,881
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	4,613		4,949
Loans and Leases Receivable, Net of Deferred Income	172,024		164,990
Loans and Leases Receivable, Allowance	4,613	4,533	4,949	3,585	3,197	3,039
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	11,271		10,165
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	82		89
Financing Receivable, Collectively Evaluated for Impairment	2,796,935		2,672,368
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	9,722		9,684
Loans and Leases Receivable, Net of Deferred Income	2,808,206		2,682,533
Loans and Leases Receivable, Allowance	9,804	9,896	9,773	10,282	10,090	10,449
Home equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	1,769		1,815
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	597		20
Financing Receivable, Collectively Evaluated for Impairment	90,032		98,143
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	739		815
Loans and Leases Receivable, Net of Deferred Income	91,801		99,958
Loans and Leases Receivable, Allowance	1,336	784	835	929	987	1,035
Consumer and other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable, Individually Evaluated for Impairment	0		125
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		125
Financing Receivable, Collectively Evaluated for Impairment	168,496		177,512
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	147		505
Loans and Leases Receivable, Net of Deferred Income	168,496		177,637
Loans and Leases Receivable, Allowance	$ 147	$ 126	$ 630	$ 405	$ 302	$ 391