Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	$ 3,783,594	$ 3,533,741
Loans, 30 to 59 Days Past Due	5,548	8,535
Loans, 60 to 89 Days Past Due	1,671	3,605
Total nonaccrual loans	13,502	22,454
Loans, Past Due, Total	20,721	34,594
Total Loans	3,804,315	3,568,335
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	1,684,411	1,595,590
Loans, 30 to 59 Days Past Due	1,606	1,209
Loans, 60 to 89 Days Past Due	458	207
Total nonaccrual loans	5,174	10,750
Loans, Past Due, Total	7,238	12,166
Total Loans	1,691,649	1,607,756
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	943,389	836,276
Loans, 30 to 59 Days Past Due	317	2,599
Loans, 60 to 89 Days Past Due	234	278
Total nonaccrual loans	2,426	3,296
Loans, Past Due, Total	2,977	6,173
Total Loans	946,366	842,449
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	181,620	139,133
Loans, 30 to 59 Days Past Due	0	1,049
Loans, 60 to 89 Days Past Due	0	751
Total nonaccrual loans	1,889	2,742
Loans, Past Due, Total	1,889	4,542
Total Loans	183,509	143,675
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	487,634	481,260
Loans, 30 to 59 Days Past Due	865	828
Loans, 60 to 89 Days Past Due	657	1,666
Total nonaccrual loans	2,248	3,338
Loans, Past Due, Total	3,770	5,832
Total Loans	491,404	487,092
Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	414,279	408,777
Loans, 30 to 59 Days Past Due	2,380	2,468
Loans, 60 to 89 Days Past Due	265	659
Total nonaccrual loans	1,735	2,291
Loans, Past Due, Total	4,380	5,418
Total Loans	418,659	414,195
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	66,140	67,420
Loans, 30 to 59 Days Past Due	380	382
Loans, 60 to 89 Days Past Due	57	44
Total nonaccrual loans	30	37
Loans, Past Due, Total	467	463
Total Loans	66,607	67,883
Consumer Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	2,995	3,149
Loans, 30 to 59 Days Past Due	0	0
Loans, 60 to 89 Days Past Due	0	0
Total nonaccrual loans	0	0
Loans, Past Due, Total	0	0
Total Loans	2,995	3,149
Loans Held for Sale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	3,126	2,136
Loans, 30 to 59 Days Past Due	0	0
Loans, 60 to 89 Days Past Due	0	0
Total nonaccrual loans	0	0
Loans, Past Due, Total	0	0
Total Loans	$ 3,126	$ 2,136