Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 5,042	$ 9,101
60-89 Days Past Due and Still Accruing, Total	966	1,202
Greater Than 90 Days Past Due and Still Accruing, Total	129	786
Total Past Due And Still Accruing, Total	6,137	11,089
Accruing Current Balances, Total	2,793,127	2,713,591
Nonaccrual Loans, Total	41,033	47,760
Total Loans	2,840,297	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		0.03%
Total Past Due And Still Accruing, % of Total Loans	0.21%	0.40%
Accruing Current Balances, % of Total Loans	98.34%	97.88%
Nonaccrual Loans, % of Total Loans	1.45%	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	257	1,214
Total Past Due And Still Accruing, Total	257	1,214
Accruing Current Balances, Total	733,348	698,416
Nonaccrual Loans, Total	6,024	4,861
Total Loans	739,629	704,491
Owner-Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total		1,264
Total Past Due And Still Accruing, Total		1,264
Accruing Current Balances, Total	755,261	755,316
Nonaccrual Loans, Total	12,681	14,001
Total Loans	767,942	770,581
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	102
Total Past Due And Still Accruing, Total	102
Accruing Current Balances, Total	677,340	618,731
Nonaccrual Loans, Total	10,791	12,634
Total Loans	688,233	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
Total Past Due And Still Accruing, Total		339
Accruing Current Balances, Total	124,892	131,489
Nonaccrual Loans, Total	646	1,547
Total Loans	125,538	133,375
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	4,012	5,383
60-89 Days Past Due and Still Accruing, Total	937	606
Greater Than 90 Days Past Due and Still Accruing, Total	129	786
Total Past Due And Still Accruing, Total	5,078	6,775
Accruing Current Balances, Total	220,661	226,863
Nonaccrual Loans, Total	7,977	9,989
Total Loans	233,716	243,627
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due and Still Accruing, Total	671	971
60-89 Days Past Due and Still Accruing, Total	29	526
Total Past Due And Still Accruing, Total	700	1,497
Accruing Current Balances, Total	281,625	282,776
Nonaccrual Loans, Total	2,914	4,728
Total Loans	$ 285,239	$ 289,001