Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 5,703,584	$ 5,730,074
Non - performing	21,335	23,938
Past due	30,312	35,860
Total Loans	5,733,896	5,765,934
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,755,336	2,681,395
Non - performing	3,952	3,468
Past due	5,555	4,599
Total Loans	2,760,891	2,685,994
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,401,154	1,426,754
Non - performing	4,617	5,646
Past due	5,796	6,512
Total Loans	1,406,950	1,433,266
Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	321,900	377,968
Non - performing	1,873	3,873
Past due	2,241	6,366
Total Loans	324,141	384,334
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	682,092	687,195
Non - performing	6,715	7,165
Past due	10,293	11,579
Total Loans	692,385	698,774
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	468,762	480,956
Non - performing	4,109	3,715
Past due	6,088	6,370
Total Loans	474,850	487,326
Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	66,551	66,770
Non - performing	69	71
Past due	339	434
Total Loans	66,890	67,204
Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,506	4,551
Non - performing	0	0
Past due	0	0
Total Loans	4,506	4,551
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,283	4,485
Non - performing	0	0
Past due	0	0
Total Loans	3,283	4,485
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	7,506	9,123
30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,162	997
30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,059	420
30 to 59 Days Past Due | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	368	2,473
30 to 59 Days Past Due | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	3,125	2,975
30 to 59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,603	2,065
30 to 59 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	189	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	1,471	2,799
60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	441	134
60 to 89 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	120	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	453	1,439
60 to 89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	376	590
60 to 89 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	81	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