Allowance for Loan Losses - Impaired Financing Receivables (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	$ 9,596		$ 9,596		$ 13,066
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	12,694		12,694		19,208
Impaired financing receivables, with no related allowance recorded, Related Allowance
Impaired financing receivables, with an allowance recorded, Recorded Investment	8,183		8,183		13,024
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	8,810		8,810		14,227
Impaired financing receivables, with an allowance recorded, Related Allowance	1,713		1,713		4,006
Impaired financing receivables, Recorded Investment, Total	17,779		17,779		26,090
Impaired financing receivables, Unpaid Principal Balance, Total	21,504		21,504		33,435
Impaired financing receivables, Related Allowance, Total	1,713		1,713		4,006
Impaired financing receivables, Average Recorded Investment, Total	20,775	27,703	23,048	26,878
Impaired financing receivables, Interest Income Recognized, Total	61	512	881	1,603
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	1,206		1,206		5,053
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	1,326		1,326		5,226
Impaired financing receivables, with no related allowance recorded, Related Allowance
Impaired financing receivables, with an allowance recorded, Recorded Investment	2,714		2,714		367
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	3,031		3,031		385
Impaired financing receivables, with an allowance recorded, Related Allowance	1,054		1,054		286
Impaired financing receivables, Recorded Investment, Total	3,920		3,920		5,420
Impaired financing receivables, Unpaid Principal Balance, Total	4,357		4,357		5,611
Impaired financing receivables, Related Allowance, Total	1,054		1,054		286
Impaired financing receivables, Average Recorded Investment, Total	4,504	5,366	4,984	4,631
Impaired financing receivables, Interest Income Recognized, Total	25	26	156	205
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	5,341		5,341		5,446
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	6,260		6,260		8,114
Impaired financing receivables, with no related allowance recorded, Related Allowance
Impaired financing receivables, with an allowance recorded, Recorded Investment	4,472		4,472		8,495
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	4,755		4,755		8,681
Impaired financing receivables, with an allowance recorded, Related Allowance	454		454		2,354
Impaired financing receivables, Recorded Investment, Total	9,813		9,813		13,941
Impaired financing receivables, Unpaid Principal Balance, Total	11,015		11,015		16,795
Impaired financing receivables, Related Allowance, Total	454		454		2,354
Impaired financing receivables, Average Recorded Investment, Total	11,237	16,448	12,328	17,060
Impaired financing receivables, Interest Income Recognized, Total	29	329	438	995
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	3,049		3,049		2,566
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	5,108		5,108		5,346
Impaired financing receivables, with no related allowance recorded, Related Allowance
Impaired financing receivables, with an allowance recorded, Recorded Investment	976		976		3,561
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	1,003		1,003		4,554
Impaired financing receivables, with an allowance recorded, Related Allowance	204		204		1,199
Impaired financing receivables, Recorded Investment, Total	4,025		4,025		6,127
Impaired financing receivables, Unpaid Principal Balance, Total	6,111		6,111		9,900
Impaired financing receivables, Related Allowance, Total	204		204		1,199
Impaired financing receivables, Average Recorded Investment, Total	4,773	5,227	5,324	4,628
Impaired financing receivables, Interest Income Recognized, Total	18	145	287	391
Real Estate Construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance					521
Impaired financing receivables, with no related allowance recorded, Related Allowance
Impaired financing receivables, with an allowance recorded, Recorded Investment					541
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance					547
Impaired financing receivables, with an allowance recorded, Related Allowance					107
Impaired financing receivables, Recorded Investment, Total					541
Impaired financing receivables, Unpaid Principal Balance, Total					1,068
Impaired financing receivables, Related Allowance, Total					107
Impaired financing receivables, Average Recorded Investment, Total	240	608	377	531
Impaired financing receivables, Interest Income Recognized, Total	(11)	11		11
Consumer and Other [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment					1
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance					1
Impaired financing receivables, with no related allowance recorded, Related Allowance
Impaired financing receivables, with an allowance recorded, Recorded Investment	21		21		60
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	21		21		60
Impaired financing receivables, with an allowance recorded, Related Allowance	1		1		60
Impaired financing receivables, Recorded Investment, Total	21		21		61
Impaired financing receivables, Unpaid Principal Balance, Total	21		21		61
Impaired financing receivables, Related Allowance, Total	1		1		60
Impaired financing receivables, Average Recorded Investment, Total	21	54	35	28
Impaired financing receivables, Interest Income Recognized, Total		$ 1		$ 1