Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	$ 25,675	$ 22,153
60-89 Days Past Due	1,331	3,358
Greater Than 90 Days	2,374	2,573
Total Past Due	29,380	28,084
Total Current	2,743,694	2,656,201
Total	2,773,074	2,684,285	2,560,890
Total 90 Days Past Due Still Accruing	123	133	187
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	134	355
60-89 Days Past Due	14
Total Past Due	148	355
Total Current	74,985	75,573
Total	75,133	75,928
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	16,781	13,787
60-89 Days Past Due	823	2,489
Greater Than 90 Days	1,973	1,876
Total Past Due	19,577	18,152
Total Current	1,714,027	1,660,362
Total	1,733,604	1,678,514
Total 90 Days Past Due Still Accruing	83	55
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	7,290	5,303
60-89 Days Past Due	51	287
Greater Than 90 Days	248	420
Total Past Due	7,589	6,010
Total Current	610,336	590,720
Total	617,925	596,730
Total 90 Days Past Due Still Accruing	23
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	1,470	2,708
60-89 Days Past Due	443	582
Greater Than 90 Days	153	277
Total Past Due	2,066	3,567
Total Current	344,346	329,546
Total	346,412	333,113
Total 90 Days Past Due Still Accruing	$ 17	$ 78