Activity in Allowance for Loan and Lease Losses by Respective Loan Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	$ 42,957	$ 49,426	$ 49,426
Provision (credit)	78	664	3,649
Charge-offs	(3,108)		(12,804)
Loan and lease recoveries	1,319	269	(2,686)
Net charge-offs	(1,789)	(5,029)	(10,118)
Balance at period end	41,246	45,061	42,957
Total loans and leases	2,565,069		2,531,128
Allowance for loans and leases to total loans and leases ratio	1.61%		1.70%
Balance of loans specifically evaluated for impairment	40,702		48,773
Allowance for loans specifically evaluated for impairment	4,015		5,149
Specific allowance to specific loans ratio	9.86%		10.56%
Balance of loans collectively evaluated	2,524,367		2,482,355
Allowance for loans collectively evaluated	37,231		37,808
Collective allowance to collective loans ratio	1.47%		1.52%
Commercial Business
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	6,495	6,727	6,727
Provision (credit)	2,505		(758)
Charge-offs	(1,857)		(1,022)
Loan and lease recoveries	113		1,548
Net charge-offs	(1,744)		526
Balance at period end	7,256		6,495
Total loans and leases	344,489		346,708
Allowance for loans and leases to total loans and leases ratio	2.11%		1.87%
Balance of loans specifically evaluated for impairment	7,887		8,984
Allowance for loans specifically evaluated for impairment	1,940		2,597
Specific allowance to specific loans ratio	24.60%		28.91%
Balance of loans collectively evaluated	336,602		337,724
Allowance for loans collectively evaluated	5,316		3,898
Collective allowance to collective loans ratio	1.58%		1.15%
Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	332	795	795
Provision (credit)	(252)		(478)
Charge-offs	0		(8)
Loan and lease recoveries	0		23
Net charge-offs	0		15
Balance at period end	80		332
Total loans and leases	1,974		3,421
Allowance for loans and leases to total loans and leases ratio	4.05%		9.70%
Balance of loans collectively evaluated	1,974		3,421
Allowance for loans collectively evaluated	80		332
Collective allowance to collective loans ratio	4.05%		9.70%
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	3,846	4,873	4,873
Provision (credit)	535		44
Charge-offs	(564)		(1,298)
Loan and lease recoveries	56		227
Net charge-offs	(508)		(1,071)
Balance at period end	3,873		3,846
Total loans and leases	353,341		356,990
Allowance for loans and leases to total loans and leases ratio	1.10%		1.08%
Balance of loans specifically evaluated for impairment	31		31
Balance of loans collectively evaluated	353,310		356,959
Allowance for loans collectively evaluated	3,873		3,846
Collective allowance to collective loans ratio	1.10%		1.08%
Commercial Real Estate Portfolio Segment | Commercial Acquisition, Development and Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	4,737	6,664	6,664
Provision (credit)	(1,831)		826
Charge-offs	0		(3,281)
Loan and lease recoveries	1,020		528
Net charge-offs	1,020		(2,753)
Balance at period end	3,926		4,737
Total loans and leases	150,599		151,933
Allowance for loans and leases to total loans and leases ratio	2.61%		3.12%
Balance of loans specifically evaluated for impairment	5,826		6,332
Allowance for loans specifically evaluated for impairment	149		0
Specific allowance to specific loans ratio	2.56%		0.00%
Balance of loans collectively evaluated	144,773		145,601
Allowance for loans collectively evaluated	3,777		4,737
Collective allowance to collective loans ratio	2.61%		3.25%
Commercial Real Estate Portfolio Segment | Commercial Investor Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	9,583	8,248	8,248
Provision (credit)	(934)		4,928
Charge-offs	(131)		(3,690)
Loan and lease recoveries	100		97
Net charge-offs	(31)		(3,593)
Balance at period end	8,618		9,583
Total loans and leases	487,802		456,888
Allowance for loans and leases to total loans and leases ratio	1.77%		2.10%
Balance of loans specifically evaluated for impairment	13,205		11,843
Allowance for loans specifically evaluated for impairment	242		774
Specific allowance to specific loans ratio	1.83%		6.54%
Balance of loans collectively evaluated	474,597		445,045
Allowance for loans collectively evaluated	8,376		8,809
Collective allowance to collective loans ratio	1.76%		1.98%
Commercial Real Estate Portfolio Segment | Commercial Owner Occupied Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	6,997	7,329	7,329
Provision (credit)	37		804
Charge-offs	(91)		(1,174)
Loan and lease recoveries	10		38
Net charge-offs	(81)		(1,136)
Balance at period end	6,953		6,997
Total loans and leases	565,820		571,510
Allowance for loans and leases to total loans and leases ratio	1.23%		1.22%
Balance of loans specifically evaluated for impairment	7,243		15,184
Allowance for loans specifically evaluated for impairment	377		598
Specific allowance to specific loans ratio	5.21%		3.94%
Balance of loans collectively evaluated	558,577		556,326
Allowance for loans collectively evaluated	6,576		6,399
Collective allowance to collective loans ratio	1.18%		1.15%
Residential Real Estate Portfolio Segment | Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	8,522	10,583	10,583
Provision (credit)	44		(167)
Charge-offs	(465)		(2,107)
Loan and lease recoveries	18		213
Net charge-offs	(447)		(1,894)
Balance at period end	8,119		8,522
Total loans and leases	538,346		523,364
Allowance for loans and leases to total loans and leases ratio	1.51%		1.63%
Balance of loans specifically evaluated for impairment	4,506		4,528
Allowance for loans specifically evaluated for impairment	698		713
Specific allowance to specific loans ratio	15.49%		15.75%
Balance of loans collectively evaluated	533,840		518,836
Allowance for loans collectively evaluated	7,421		7,809
Collective allowance to collective loans ratio	1.39%		1.51%
Residential Real Estate Portfolio Segment | Residential Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of year	2,445	4,207	4,207
Provision (credit)	(26)		(1,550)
Charge-offs	0		(224)
Loan and lease recoveries	2		12
Net charge-offs	2		(212)
Balance at period end	2,421		2,445
Total loans and leases	122,698		120,314
Allowance for loans and leases to total loans and leases ratio	1.97%		2.03%
Balance of loans specifically evaluated for impairment	2,004		1,871
Allowance for loans specifically evaluated for impairment	609		467
Specific allowance to specific loans ratio	30.39%		24.96%
Balance of loans collectively evaluated	120,694		118,443
Allowance for loans collectively evaluated	$ 1,812		$ 1,978
Collective allowance to collective loans ratio	1.50%		1.67%