Allowance for Credit Losses and Loan Portfolio by Loan Segment (Detail) (USD $)	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 6,076,000		$ 7,938,000
Collectively evaluated for impairment	64,067,000		62,960,000
Total Allowance for Loan Losses	70,143,000	70,369,000	70,898,000	77,133,000	80,936,000	82,977,000
Individually evaluated for impairment	88,282,000		83,319,000
Collectively evaluated for impairment	2,709,352,000		2,630,096,000
Loans	2,797,634,000		2,713,415,000
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,447,000		4,701,000
Collectively evaluated for impairment	15,344,000		13,030,000
Total Allowance for Loan Losses	17,791,000	15,574,000	17,731,000	23,715,000	30,206,000	32,508,000
Individually evaluated for impairment	16,596,000		18,793,000
Collectively evaluated for impairment	687,260,000		657,760,000
Loans	703,856,000		676,553,000
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,202,000		2,504,000
Collectively evaluated for impairment	32,931,000		35,415,000
Total Allowance for Loan Losses	35,133,000	37,907,000	37,919,000	38,668,000	37,240,000	36,341,000
Individually evaluated for impairment	56,343,000		51,980,000
Collectively evaluated for impairment	1,262,359,000		1,224,031,000
Loans	1,318,702,000		1,276,011,000
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	2,431,000		2,902,000
Total Allowance for Loan Losses	2,431,000	2,805,000	2,902,000	2,859,000	3,098,000	3,622,000
Individually evaluated for impairment	139,000
Collectively evaluated for impairment	83,794,000		84,172,000
Loans	83,933,000		84,172,000
Residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,427,000		733,000
Collectively evaluated for impairment	13,361,000		11,610,000
Total Allowance for Loan Losses	14,788,000	14,083,000	12,343,000	11,877,000	10,371,000	10,408,000
Individually evaluated for impairment	15,204,000		12,546,000
Collectively evaluated for impairment	672,963,000		660,578,000
Loans	688,167,000		673,124,000
Finance Lease
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment			3,000
Total Allowance for Loan Losses			3,000	14,000	21,000	98,000
Collectively evaluated for impairment	2,976,000		3,555,000
Loans	$ 2,976,000		$ 3,555,000