Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	$ 5,829,789	$ 5,563,123
Total nonaccrual loans	29,493	42,635
Total past due	38,905	52,089
Total Loans	5,868,694	5,615,212
Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	2,674,171	2,479,513
Total nonaccrual loans	6,571	16,172
Total past due	7,522	18,963
Total Loans	2,681,693	2,498,476
Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	1,438,224	1,391,475
Total nonaccrual loans	7,349	8,071
Total past due	8,587	9,560
Total Loans	1,446,811	1,401,035
Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	428,744	450,410
Total nonaccrual loans	4,068	4,927
Total past due	4,143	5,474
Total Loans	432,887	455,884
Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	684,848	689,635
Total nonaccrual loans	7,782	9,918
Total past due	12,519	12,347
Total Loans	697,367	701,982
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	482,000	476,866
Total nonaccrual loans	3,675	3,439
Total past due	5,806	5,418
Total Loans	487,806	482,284
Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	69,316	65,525
Total nonaccrual loans	48	108
Total past due	328	327
Total Loans	69,644	65,852
Consumer Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	4,550	5,906
Total nonaccrual loans	0	0
Total past due	0	0
Total Loans	4,550	5,906
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	47,936	3,793
Total nonaccrual loans	0	0
Total past due	0	0
Total Loans	47,936	3,793
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6,820	6,598
30 to 59 Days Past Due | Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	530	2,032
30 to 59 Days Past Due | Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	499	1,061
30 to 59 Days Past Due | Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	75	547
30 to 59 Days Past Due | Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,872	1,312
30 to 59 Days Past Due | Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,616	1,470
30 to 59 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	228	176
30 to 59 Days Past Due | Consumer Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
30 to 59 Days Past Due | Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,592	2,856
60 to 89 Days Past Due | Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	421	759
60 to 89 Days Past Due | Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	739	428
60 to 89 Days Past Due | Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
60 to 89 Days Past Due | Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	865	1,117
60 to 89 Days Past Due | Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	515	509
60 to 89 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	52	43
60 to 89 Days Past Due | Consumer Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
60 to 89 Days Past Due | Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	$ 0