Fair Value - Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
ASSETS
Cash and cash equivalents	$ 27,418	$ 27,928	$ 29,397	$ 29,633
Interest bearing time deposits with other banks	225	225
Securities available for sale	587,333	685,720
Trading securities	4,489	4,505
Loans held for sale	551	887
Net loans	1,498,885	1,337,916
FHLB and other equity interests	13,438	6,695
LIABILITIES
Deposits	(1,847,692)	(1,847,079)
Subordinated debentures	(20,620)	(20,620)
Carrying Amount [Member]
ASSETS
Cash and cash equivalents	27,418	27,928
Interest bearing time deposits with other banks	225	225
Securities available for sale	587,333	685,720
Trading securities	4,489	4,505
Loans held for sale	551	887
Net loans	1,498,885	1,337,916
FHLB and other equity interests	13,438	6,695
Accrued interest receivable	7,872	7,441
LIABILITIES
Deposits	(1,847,692)	(1,847,079)
FHLB and other borrowings	(166,030)	(111,695)
Subordinated debentures	(20,620)	(20,620)
Interest rate swaps	(908)	(946)
Accrued interest payable	(918)	(804)
Total Fair Value [Member]
ASSETS
Cash and cash equivalents	27,418	27,928
Interest bearing time deposits with other banks	225	224
Securities available for sale	587,333	685,720
Trading securities	4,489	4,505
Loans held for sale	572	938
Net loans	1,498,395	1,337,537
Accrued interest receivable	7,872	7,441
LIABILITIES
Deposits	(1,847,831)	(1,846,208)
FHLB and other borrowings	(165,265)	(111,821)
Subordinated debentures	(11,524)	(11,395)
Interest rate swaps	(908)	(946)
Accrued interest payable	(918)	(804)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
ASSETS
Cash and cash equivalents	27,418	27,928
Interest bearing time deposits with other banks	0	0
Securities available for sale	993	1,002
Trading securities	4,279	4,296
Loans held for sale	0	0
Net loans	0	0
Accrued interest receivable	6	5
LIABILITIES
Deposits	(1,650,797)	(1,670,172)
FHLB and other borrowings	0	0
Subordinated debentures	0	0
Interest rate swaps	0	0
Accrued interest payable	(357)	(358)
Significant Other Observable Inputs (Level 2) [Member]
ASSETS
Cash and cash equivalents	0	0
Interest bearing time deposits with other banks	225	224
Securities available for sale	585,180	683,813
Trading securities	210	209
Loans held for sale	572	938
Net loans	0	0
Accrued interest receivable	3,680	3,503
LIABILITIES
Deposits	(197,034)	(176,036)
FHLB and other borrowings	(165,265)	(111,821)
Subordinated debentures	(11,524)	(11,395)
Interest rate swaps	(908)	(946)
Accrued interest payable	(545)	(430)
Significant Unobservable Inputs (Level 3) [Member]
ASSETS
Cash and cash equivalents	0	0
Interest bearing time deposits with other banks	0	0
Securities available for sale	1,160	905
Trading securities	0	0
Loans held for sale	0	0
Net loans	1,498,395	1,337,537
Accrued interest receivable	4,186	3,933
LIABILITIES
Deposits	0	0
FHLB and other borrowings	0	0
Subordinated debentures	0	0
Interest rate swaps	0	0
Accrued interest payable	$ (16)	$ (16)