Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	$ 5,372,840	$ 5,002,925
Total nonaccrual loans	40,493	35,382
Total past due	57,088	60,008
Total Loans	5,429,928	5,062,933
Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	1,327,898	1,244,802
Total nonaccrual loans	11,890	9,279
Total past due	16,399	12,028
Total Loans	1,344,297	1,256,830
Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	2,413,004	2,145,655
Total nonaccrual loans	7,551	8,719
Total past due	14,160	20,948
Total Loans	2,427,164	2,166,603
Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	394,812	401,084
Total nonaccrual loans	6,653	8,753
Total past due	7,312	12,360
Total Loans	402,124	413,444
Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	679,237	631,085
Total nonaccrual loans	11,400	5,629
Total past due	13,337	8,287
Total Loans	692,574	639,372
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	478,389	465,055
Total nonaccrual loans	2,955	2,902
Total past due	5,546	5,790
Total Loans	483,935	470,845
Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	61,954	73,486
Total nonaccrual loans	44	100
Total past due	334	453
Total Loans	62,288	73,939
Consumer Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	5,852	6,579
Total nonaccrual loans	0	0
Total past due	0	0
Total Loans	5,852	6,579
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	11,694	35,179
Total nonaccrual loans	0	0
Total past due	0	142
Total Loans	11,694	35,321
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	10,389	22,001
30 to 59 Days Past Due | Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,536	2,453
30 to 59 Days Past Due | Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,286	11,602
30 to 59 Days Past Due | Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	456	3,517
30 to 59 Days Past Due | Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	585	1,728
30 to 59 Days Past Due | Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,321	2,365
30 to 59 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	205	242
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	94
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6,206	2,625
60 to 89 Days Past Due | Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	973	296
60 to 89 Days Past Due | Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,323	627
60 to 89 Days Past Due | Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	203	90
60 to 89 Days Past Due | Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,352	930
60 to 89 Days Past Due | Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	270	523
60 to 89 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	85	111
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	$ 48