Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 4,963	$ 9,101
60-89 Days Past Due and Still Accruing, Total	705	1,202
Greater Than 90 Days Past Due and Still Accruing, Total	400	786
Total Past Due And Still Accruing, Total	6,068	11,089
Accruing Current Balances, Total	2,735,648	2,713,591
Nonaccrual Loans, Total	45,721	47,760
Total Loans	2,787,437	2,772,440
30-59 Days Past Due and Still Accruing, % of Total Loans	0.18%	0.33%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.03%	0.04%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.01%	0.03%
Total Past Due And Still Accruing, % of Total Loans	0.22%	0.40%
Accruing Current Balances, % of Total Loans	98.14%	97.88%
Nonaccrual Loans, % of Total Loans	1.64%	1.72%
% of Total Loans	100.00%	100.00%
Commercial Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	900	1,214
60-89 Days Past Due and Still Accruing, Total	223
Greater Than 90 Days Past Due and Still Accruing, Total	103
Total Past Due And Still Accruing, Total	1,226	1,214
Accruing Current Balances, Total	726,605	698,416
Nonaccrual Loans, Total	4,991	4,861
Total Loans	732,822	704,491
Owner-Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	511	1,264
60-89 Days Past Due and Still Accruing, Total
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total	511	1,264
Accruing Current Balances, Total	752,450	755,316
Nonaccrual Loans, Total	13,263	14,001
Total Loans	766,224	770,581
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total
60-89 Days Past Due and Still Accruing, Total
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total
Accruing Current Balances, Total	622,510	618,731
Nonaccrual Loans, Total	11,240	12,634
Total Loans	633,750	631,365
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total		269
60-89 Days Past Due and Still Accruing, Total		70
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total		339
Accruing Current Balances, Total	132,265	131,489
Nonaccrual Loans, Total	1,216	1,547
Total Loans	133,481	133,375
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	2,925	5,383
60-89 Days Past Due and Still Accruing, Total	315	606
Greater Than 90 Days Past Due and Still Accruing, Total	297	786
Total Past Due And Still Accruing, Total	3,537	6,775
Accruing Current Balances, Total	223,507	226,863
Nonaccrual Loans, Total	10,810	9,989
Total Loans	237,854	243,627
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	627	971
60-89 Days Past Due and Still Accruing, Total	167	526
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total	794	1,497
Accruing Current Balances, Total	278,311	282,776
Nonaccrual Loans, Total	4,201	4,728
Total Loans	$ 283,306	$ 289,001