Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	$ 5,102,365	$ 5,002,925
Total nonaccrual loans	51,834	35,382
Total past due	86,122	60,008
Total Loans	5,188,487	5,062,933
Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	2,238,144	2,145,655
Total nonaccrual loans	15,244	8,719
Total past due	22,087	20,948
Total Loans	2,260,231	2,166,603
Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	1,306,834	1,244,802
Total nonaccrual loans	14,209	9,279
Total past due	27,285	12,028
Total Loans	1,334,119	1,256,830
Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	364,675	401,084
Total nonaccrual loans	9,993	8,753
Total past due	14,618	12,360
Total Loans	379,293	413,444
Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	636,328	631,085
Total nonaccrual loans	9,012	5,629
Total past due	14,216	8,287
Total Loans	650,544	639,372
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	460,203	465,055
Total nonaccrual loans	3,267	2,902
Total past due	7,468	5,790
Total Loans	467,671	470,845
Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	75,741	73,486
Total nonaccrual loans	109	100
Total past due	448	453
Total Loans	76,189	73,939
Consumer Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	8,701	6,579
Total nonaccrual loans	0	0
Total past due	0	0
Total Loans	8,701	6,579
Loans Held for Sale
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	11,739	35,179
Total nonaccrual loans	0	0
Total past due	0	142
Total Loans	11,739	35,321
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	26,064	22,001
30 to 59 Days Past Due | Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6,669	11,602
30 to 59 Days Past Due | Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	12,509	2,453
30 to 59 Days Past Due | Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,086	3,517
30 to 59 Days Past Due | Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,749	1,728
30 to 59 Days Past Due | Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,754	2,365
30 to 59 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	297	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	8,224	2,625
60 to 89 Days Past Due | Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	174	627
60 to 89 Days Past Due | Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	567	296
60 to 89 Days Past Due | Commercial Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,539	90
60 to 89 Days Past Due | Residential Mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,455	930
60 to 89 Days Past Due | Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	447	523
60 to 89 Days Past Due | Installment and Other Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	42	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