Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	$ 3,597,570	$ 3,531,605
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	8,521	8,535
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	783	3,605
Total nonaccrual loans	21,022	22,454
Financing Receivable, Recorded Investment, Past Due, Total	30,326	34,594
Total Portfolio Loans	3,627,896	3,566,199
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	1,596,756	1,595,590
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	937	1,209
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	0	207
Total nonaccrual loans	10,265	10,750
Financing Receivable, Recorded Investment, Past Due, Total	11,202	12,166
Total Portfolio Loans	1,607,958	1,607,756
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	880,554	836,276
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	850	2,599
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	285	278
Total nonaccrual loans	3,181	3,296
Financing Receivable, Recorded Investment, Past Due, Total	4,316	6,173
Total Portfolio Loans	884,870	842,449
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	162,142	139,133
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	3,314	1,049
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	0	751
Total nonaccrual loans	1,976	2,742
Financing Receivable, Recorded Investment, Past Due, Total	5,290	4,542
Total Portfolio Loans	167,432	143,675
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	485,205	481,260
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	1,757	828
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	210	1,666
Total nonaccrual loans	2,948	3,338
Financing Receivable, Recorded Investment, Past Due, Total	4,915	5,832
Total Portfolio Loans	490,120	487,092
Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	406,413	408,777
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	1,409	2,468
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	243	659
Total nonaccrual loans	2,630	2,291
Financing Receivable, Recorded Investment, Past Due, Total	4,282	5,418
Total Portfolio Loans	410,695	414,195
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	64,240	67,420
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	254	382
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	45	44
Total nonaccrual loans	22	37
Financing Receivable, Recorded Investment, Past Due, Total	321	463
Total Portfolio Loans	64,561	67,883
Consumer Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	2,260	3,149
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	0	0
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	0	0
Total nonaccrual loans	0	0
Financing Receivable, Recorded Investment, Past Due, Total	0	0
Total Portfolio Loans	$ 2,260	$ 3,149