Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	$ 19,372	$ 22,153
60-89 Days Past Due	1,339	3,358
Greater Than 90 Days	2,177	2,573
Total Past Due	22,888	28,084
Total Current	2,906,300	2,656,201
Total	2,929,188	2,684,285
Total 90 Days Past Due Still Accruing	261	133
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	437	355
60-89 Days Past Due	42
Total Past Due	479	355
Total Current	105,215	75,573
Total	105,694	75,928
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	12,002	13,787
60-89 Days Past Due	707	2,489
Greater Than 90 Days	1,868	1,876
Total Past Due	14,577	18,152
Total Current	1,808,277	1,660,362
Total	1,822,854	1,678,514
Total 90 Days Past Due Still Accruing	207	55
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	4,611	5,303
60-89 Days Past Due	94	287
Greater Than 90 Days	175	420
Total Past Due	4,880	6,010
Total Current	635,074	590,720
Total	639,954	596,730
Total 90 Days Past Due Still Accruing	24
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	2,322	2,708
60-89 Days Past Due	496	582
Greater Than 90 Days	134	277
Total Past Due	2,952	3,567
Total Current	357,734	329,546
Total	360,686	333,113
Total 90 Days Past Due Still Accruing	$ 30	$ 78