Composition of Impaired Loans by Loan Class (Detail) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 108,856,000	$ 108,856,000	$ 98,922,000
Unpaid Principal Balance	129,341,000	129,341,000	123,156,000
Recorded Investment	62,881,000	62,881,000	59,835,000
Recorded Investment	81,241,000	81,241,000	79,775,000
Related Allowance
Average Recorded Investment	67,077,000	70,429,000	69,330,000
Interest Income Recognized	388,000	700,000	1,241,000
Unpaid Principal Balance	20,485,000	20,485,000	24,234,000
Recorded Investment	18,360,000	18,360,000	19,940,000
Related Allowance	5,201,000	5,201,000	7,596,000
Average Recorded Investment	18,423,000	18,570,000	21,462,000
Average Recorded Investment	85,500,000	88,999,000	90,792,000
Interest Income Recognized	75,000	148,000	321,000
Interest Income Recognized	463,000	848,000	1,562,000
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	26,307,000	26,307,000	23,364,000
Recorded Investment	10,487,000	10,487,000	10,116,000
Related Allowance
Average Recorded Investment	11,411,000	12,418,000	13,399,000
Interest Income Recognized	39,000	64,000	615,000
Unpaid Principal Balance	6,523,000	6,523,000	8,691,000
Recorded Investment	6,091,000	6,091,000	8,104,000
Related Allowance	2,447,000	2,447,000	4,142,000
Average Recorded Investment	6,111,000	6,136,000	8,196,000
Interest Income Recognized	11,000	21,000	174,000
Construction
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	12,309,000	12,309,000	14,301,000
Recorded Investment	7,197,000	7,197,000	7,701,000
Related Allowance
Average Recorded Investment	8,040,000	8,581,000	8,836,000
Interest Income Recognized	16,000	29,000
Unpaid Principal Balance	2,176,000	2,176,000	961,000
Recorded Investment	1,931,000	1,931,000	961,000
Related Allowance	235,000	235,000	321,000
Average Recorded Investment	1,931,000	1,936,000	961,000
Commercial and farm land
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	57,740,000	57,740,000	49,242,000
Recorded Investment	38,884,000	38,884,000	34,571,000
Related Allowance
Average Recorded Investment	41,084,000	42,615,000	39,032,000
Interest Income Recognized	315,000	575,000	591,000
Unpaid Principal Balance	9,671,000	9,671,000	12,115,000
Recorded Investment	8,330,000	8,330,000	8,748,000
Related Allowance	1,967,000	1,967,000	2,183,000
Average Recorded Investment	8,369,000	8,505,000	10,028,000
Interest Income Recognized	45,000	89,000	140,000
Residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	8,372,000	8,372,000	7,491,000
Recorded Investment	5,589,000	5,589,000	6,185,000
Related Allowance
Average Recorded Investment	5,815,000	6,072,000	6,539,000
Interest Income Recognized	15,000	26,000	20,000
Unpaid Principal Balance	2,115,000	2,115,000	1,888,000
Recorded Investment	2,008,000	2,008,000	1,575,000
Related Allowance	552,000	552,000	391,000
Average Recorded Investment	2,012,000	1,993,000	1,687,000
Interest Income Recognized	19,000	38,000	7,000
Home Equity
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	3,754,000	3,754,000	4,425,000
Recorded Investment	567,000	567,000	1,241,000
Related Allowance
Average Recorded Investment	570,000	585,000	1,500,000
Interest Income Recognized	3,000	6,000	15,000
Consumer
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	283,000	283,000
Recorded Investment	139,000	139,000
Related Allowance
Average Recorded Investment	139,000	139,000
Other Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	91,000	91,000	99,000
Recorded Investment	18,000	18,000	21,000
Related Allowance
Average Recorded Investment	18,000	19,000	24,000
Unpaid Principal Balance			579,000
Recorded Investment			552,000
Related Allowance			559,000
Average Recorded Investment			$ 590,000