Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 5,878,384	$ 5,890,141
Past due	59,774	58,878
Non - performing	47,950	46,073
Total Loans	5,938,158	5,949,019
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,868,403	2,903,997
Past due	33,222	17,835
Non - performing	29,109	12,052
Total Loans	2,901,625	2,921,832
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,504,824	1,482,473
Past due	8,183	10,943
Non - performing	6,810	8,960
Total Loans	1,513,007	1,493,416
Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	244,432	243,004
Past due	1,226	14,193
Non - performing	1,226	14,193
Total Loans	245,658	257,197
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	719,422	717,447
Past due	10,492	9,232
Non - performing	6,630	7,128
Total Loans	729,914	726,679
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	457,452	465,152
Past due	6,114	6,410
Non - performing	4,146	3,698
Total Loans	463,566	471,562
Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	70,645	67,281
Past due	315	265
Non - performing	29	42
Total Loans	70,960	67,546
Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,500	8,416
Past due	222	0
Non - performing	0	0
Total Loans	10,722	8,416
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,706	2,371
Past due	0	0
Non - performing	0	0
Total Loans	2,706	2,371
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	8,519	8,473
30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,876	3,638
30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	785	1,000
30 to 59 Days Past Due | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	0
30 to 59 Days Past Due | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	3,695	1,584
30 to 59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,714	2,103
30 to 59 Days Past Due | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	227	148
30 to 59 Days Past Due | Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	222	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	3,305	4,332
60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,237	2,145
60 to 89 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	588	983
60 to 89 Days Past Due | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	0	0
60 to 89 Days Past Due | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	167	520
60 to 89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	254	609
60 to 89 Days Past Due | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	59	75
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