Loans and Allowance for Loan and Lease Losses - Summary of Activity in ALLL (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 12,639	$ 11,940	$ 13,833	$ 11,358
Charge offs	(881)	(168)	(2,353)	(378)
Recoveries	585	208	863	550
Provision	0	600	0	1,050
Balance at end of period	12,343	12,580	12,343	12,580
Commercial loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	7,353	5,988	7,670	5,812
Charge offs	(730)	(166)	(2,117)	(276)
Recoveries	582	169	857	452
Provision	(724)	486	71	489
Balance at end of period	6,481	6,477	6,481	6,477
Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	4,287	5,039	5,133	4,517
Charge offs	0	0	0	0
Recoveries	1	1	2	19
Provision	601	357	(246)	861
Balance at end of period	4,889	5,397	4,889	5,397
Land Development [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	120	154	296	165
Charge offs	0	0	(85)	0
Recoveries	0	0	0	0
Provision	(76)	(69)	(167)	(80)
Balance at end of period	44	85	44	85
Consumer and Single Family Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	879	759	734	864
Charge offs	(151)	(2)	(151)	(102)
Recoveries	2	38	4	79
Provision	199	(174)	342	(220)
Balance at end of period	$ 929	$ 621	$ 929	$ 621