Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 5,524	$ 5,115
60-89 Days Past Due and Still Accruing, Total	175	2,574
Greater Than 90 Days Past Due and Still Accruing, Total		533
Total Past Due And Still Accruing, Total	5,699	8,222
Accruing Current Balances, Total	3,001,717	2,913,093
Nonaccrual Loans, Total	34,061	30,948
Total Loans	3,041,477	2,952,263
30-59 Days Past Due and Still Accruing, % of Total Loans	0.18%	0.17%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.01%	0.09%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans		0.02%
Total Past Due And Still Accruing, % of Total Loans	0.19%	0.28%
Accruing Current Balances, % of Total Loans	98.69%	98.67%
Nonaccrual Loans, % of Total Loans	1.12%	1.05%
% of Total Loans	100.00%	100.00%
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	560	1,447
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	560	1,447
Accruing Current Balances, Total	853,283	805,132
Nonaccrual Loans, Total	3,511	4,303
Total Loans	857,354	810,882
Owner-Occupied Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,953	538
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	1,953	538
Accruing Current Balances, Total	771,376	780,625
Nonaccrual Loans, Total	4,928	5,197
Total Loans	778,257	786,360
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total		83
60-89 Days Past Due and Still Accruing, Total		1,049
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total		1,132
Accruing Current Balances, Total	749,699	715,496
Nonaccrual Loans, Total	13,391	8,565
Total Loans	763,090	725,193
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [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	119,333	104,916
Nonaccrual Loans, Total		1,158
Total Loans	119,333	106,074
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	2,395	1,952
60-89 Days Past Due and Still Accruing, Total	41	1,348
Greater Than 90 Days Past Due and Still Accruing, Total		533
Total Past Due And Still Accruing, Total	2,436	3,833
Accruing Current Balances, Total	200,002	209,255
Nonaccrual Loans, Total	8,485	8,432
Total Loans	210,923	221,520
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	616	1,095
60-89 Days Past Due and Still Accruing, Total	134	177
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total	750	1,272
Accruing Current Balances, Total	308,024	297,669
Nonaccrual Loans, Total	3,746	3,293
Total Loans	$ 312,520	$ 302,234