Allowance for Loan Losses - Credit Exposures by Internally Assigned Grades (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	$ 669,220	$ 659,229
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	623,772	595,300
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	11,535	10,320
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	33,913	53,609
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	105,830	100,661
Commercial and Agriculture [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	97,258	90,159
Commercial and Agriculture [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	2,804	1,653
Commercial and Agriculture [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	5,768	8,849
Commercial and 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	431,733	434,808
Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	406,253	397,657
Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	7,714	6,371
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	17,766	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	106,962	104,713
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	95,677	89,896
Residential Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	996	1,944
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	10,289	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	22,316	17,947
Real Estate Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	22,295	16,594
Real Estate Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	21	352
Real Estate Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades		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	2,379	1,100
Consumer [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades	2,289	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	90	106
Consumer [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit exposures by internally assigned grades