Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 5,781,619	$ 5,730,074
Non - performing	20,739	23,938
Past due	30,395	35,860
Total Loans	5,812,014	5,765,934
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,820,689	2,681,395
Non - performing	4,595	3,468
Past due	5,683	4,599
Total Loans	2,826,372	2,685,994
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,445,433	1,426,754
Non - performing	4,367	5,646
Past due	5,938	6,512
Total Loans	1,451,371	1,433,266
Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	282,489	377,968
Non - performing	1,228	3,873
Past due	1,294	6,366
Total Loans	283,783	384,334
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	689,464	687,195
Non - performing	6,721	7,165
Past due	10,403	11,579
Total Loans	699,867	698,774
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	465,625	480,956
Non - performing	3,783	3,715
Past due	6,826	6,370
Total Loans	472,451	487,326
Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	67,291	66,770
Non - performing	45	71
Past due	251	434
Total Loans	67,542	67,204
Consumer construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	6,421	4,551
Non - performing	0	0
Past due	0	0
Total Loans	6,421	4,551
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,207	4,485
Non - performing	0	0
Past due	0	0
Total Loans	4,207	4,485
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	7,097	9,123
30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	664	997
30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,400	420
30 to 59 Days Past Due | Commercial construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	66	2,473
30 to 59 Days Past Due | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,242	2,975
30 to 59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,590	2,065
30 to 59 Days Past Due | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	135	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	2,559	2,799
60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	424	134
60 to 89 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	171	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	1,440	1,439
60 to 89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	453	590
60 to 89 Days Past Due | Installment and other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	71	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