Aging of Recorded Investment in Past Due Loans by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	$ 1,213,423	[1]	$ 1,129,305	[2]
30-59 Days Past Due	4,508		3,826
60-89 Days Past Due	713		1,164
90 Days or More Past Due	2,964		8,120
Total Past Due	8,185		13,110
Loans Not Past Due	1,205,238		1,116,195
Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	336,307	[1]	294,087	[2]
30-59 Days Past Due	436		220
60-89 Days Past Due	133		0
90 Days or More Past Due	448		1,141
Total Past Due	1,017		1,361
Loans Not Past Due	335,290		292,726
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	489,796	[1]	453,318	[2]
30-59 Days Past Due	1,352		381
60-89 Days Past Due	0		148
90 Days or More Past Due	2,063		5,920
Total Past Due	3,415		6,449
Loans Not Past Due	486,381		446,869
Agricultural Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	182,575	[1]	170,513	[2]
30-59 Days Past Due	42		10
60-89 Days Past Due	14		0
90 Days or More Past Due	0		0
Total Past Due	56		10
Loans Not Past Due	182,519		170,503
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	74,699	[1]	77,323	[2]
30-59 Days Past Due	177		176
60-89 Days Past Due	48		6
90 Days or More Past Due	178		90
Total Past Due	403		272
Loans Not Past Due	74,296		77,051
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	41,231	[1]	47,595	[2]
30-59 Days Past Due	431		287
60-89 Days Past Due	23		117
90 Days or More Past Due	18		221
Total Past Due	472		625
Loans Not Past Due	40,759		46,970
Residential Mortgage Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	88,815	[1]	86,469	[2]
30-59 Days Past Due	2,070		2,752
60-89 Days Past Due	495		893
90 Days or More Past Due	257		748
Total Past Due	2,822		4,393
Loans Not Past Due	85,993		82,076
Loans Acquired With Deteriorated Credit Quality [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Ending Loans Balance (1)	11,174		16,121
30-59 Days Past Due	0		248
60-89 Days Past Due	120		56
90 Days or More Past Due	0		554
Total Past Due	120		858
Loans Not Past Due	$ 11,054		$ 15,263

[1] Total recorded investment in loans includes $5,522 in accrued interest.

[2] Total recorded investment in loans includes $5,756 in accrued interest.