Allowance for Loan Losses - Credit Exposures by Internally Assigned Grades (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	$ 657,583	$ 659,229
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	599,397	595,300
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	7,877	10,320
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	50,309	53,609
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Commercial & Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	110,551	100,661
Commercial & Agriculture [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	100,167	90,159
Commercial & Agriculture [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	2,151	1,653
Commercial & Agriculture [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	8,233	8,849
Commercial & Agriculture [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	421,265	434,808
Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	390,243	397,657
Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	5,002	6,371
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	26,020	30,780
Commercial Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	105,952	104,713
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	90,012	89,896
Residential Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	724	1,944
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	15,216	12,873
Residential Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	19,001	17,947
Real Estate Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	18,235	16,594
Real Estate Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades		352
Real Estate Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	766	1,001
Real Estate Construction [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	814	1,100
Consumer [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	740	994
Consumer [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Consumer [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	74	106
Consumer [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades