Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 5,759,858	$ 5,730,074
Non - performing	21,372	23,938
Past due	30,061	35,860
Total Loans	5,789,919	5,765,934
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,781,239	2,681,395
Non - performing	4,617	3,468
Past due	7,402	4,599
Total Loans	2,788,641	2,685,994
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,450,174	1,426,754
Non - performing	4,853	5,646
Past due	5,404	6,512
Total Loans	1,455,578	1,433,266
Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	297,849	377,968
Non - performing	1,870	3,873
Past due	1,938	6,366
Total Loans	299,787	384,334
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	689,751	687,195
Non - performing	6,112	7,165
Past due	8,689	11,579
Total Loans	698,440	698,774
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	465,261	480,956
Non - performing	3,871	3,715
Past due	6,361	6,370
Total Loans	471,622	487,326
Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	66,371	66,770
Non - performing	49	71
Past due	267	434
Total Loans	66,638	67,204
Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	5,412	4,551
Non - performing	0	0
Past due	0	0
Total Loans	5,412	4,551
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,801	4,485
Non - performing	0	0
Past due	0	0
Total Loans	3,801	4,485
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	6,496	9,123
30 to 59 Days Past Due \| Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,473	997
30 to 59 Days Past Due \| Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	432	420
30 to 59 Days Past Due \| Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	68	2,473
30 to 59 Days Past Due \| Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,445	2,975
30 to 59 Days Past Due \| Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,906	2,065
30 to 59 Days Past Due \| Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	172	193
30 to 59 Days Past Due \| Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	0	0
30 to 59 Days Past Due \| Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	0	0
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,193	2,799
60 to 89 Days Past Due \| Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	312	134
60 to 89 Days Past Due \| Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	119	446
60 to 89 Days Past Due \| Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	0	20
60 to 89 Days Past Due \| Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,132	1,439
60 to 89 Days Past Due \| Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	584	590
60 to 89 Days Past Due \| Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	46	170
60 to 89 Days Past Due \| Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	0	0
60 to 89 Days Past Due \| Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	$ 0	$ 0

Financing receivables that are current.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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