Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	$ 19,755,000	$ 19,372,000
60-89 Days Past Due	1,824,000	1,339,000
Greater Than 90 Days	1,794,000	2,177,000
Total Past Due	23,373,000	22,888,000
Total Current	2,905,103,000	2,906,300,000
Total	2,928,476,000	2,929,188,000	2,688,288,000
Total 90 Days Past Due Still Accruing	184,000	261,000	30,000
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	4,514,000	4,611,000
60-89 Days Past Due	283,000	94,000
Greater Than 90 Days	208,000	175,000
Total Past Due	5,005,000	4,880,000
Total Current	646,718,000	635,074,000
Total	651,723,000	639,954,000
Total 90 Days Past Due Still Accruing		24,000
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	485,000	437,000
60-89 Days Past Due	35,000	42,000
Total Past Due	520,000	479,000
Total Current	90,090,000	105,215,000
Total	90,610,000	105,694,000
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	13,077,000	12,002,000
60-89 Days Past Due	1,116,000	707,000
Greater Than 90 Days	1,368,000	1,868,000
Total Past Due	15,561,000	14,577,000
Total Current	1,811,018,000	1,808,277,000
Total	1,826,579,000	1,822,854,000
Total 90 Days Past Due Still Accruing	144,000	207,000
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	1,679,000	2,322,000
60-89 Days Past Due	390,000	496,000
Greater Than 90 Days	218,000	134,000
Total Past Due	2,287,000	2,952,000
Total Current	357,277,000	357,734,000
Total	359,564,000	360,686,000
Total 90 Days Past Due Still Accruing	$ 40,000	$ 30,000