Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 3,312	$ 5,115
60-89 Days Past Due and Still Accruing, Total	791	2,574
Greater Than 90 Days Past Due and Still Accruing, Total	403	533
Total Past Due And Still Accruing, Total	4,506	8,222
Accruing Current Balances, Total	2,960,038	2,913,093
Nonaccrual Loans, Total	40,128	30,948
Total Loans	3,004,672	2,952,263
30-59 Days Past Due and Still Accruing, % of Total Loans	0.11%	0.17%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.03%	0.09%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.01%	0.02%
Total Past Due And Still Accruing, % of Total Loans	0.15%	0.28%
Accruing Current Balances, % of Total Loans	98.51%	98.67%
Nonaccrual Loans, % of Total Loans	1.34%	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	788	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	788	1,447
Accruing Current Balances, Total	833,364	805,132
Nonaccrual Loans, Total	4,029	4,303
Total Loans	838,181	810,882
Owner-Occupied Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total		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		538
Accruing Current Balances, Total	792,895	780,625
Nonaccrual Loans, Total	6,318	5,197
Total Loans	799,213	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	730,637	715,496
Nonaccrual Loans, Total	15,692	8,565
Total Loans	746,329	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	100,415	104,916
Nonaccrual Loans, Total	1,070	1,158
Total Loans	101,485	106,074
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,788	1,952
60-89 Days Past Due and Still Accruing, Total	438	1,348
Greater Than 90 Days Past Due and Still Accruing, Total	403	533
Total Past Due And Still Accruing, Total	2,629	3,833
Accruing Current Balances, Total	206,153	209,255
Nonaccrual Loans, Total	9,371	8,432
Total Loans	218,153	221,520
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	736	1,095
60-89 Days Past Due and Still Accruing, Total	353	177
Greater Than 90 Days Past Due and Still Accruing, Total
Total Past Due And Still Accruing, Total	1,089	1,272
Accruing Current Balances, Total	296,574	297,669
Nonaccrual Loans, Total	3,648	3,293
Total Loans	$ 301,311	$ 302,234