Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 38,193	$ 22,888	$ 24,386
Total Current	3,228,624	2,906,300	2,804,044
Total	3,266,817	2,929,188	2,828,430
Total 90 Days Past Due Still Accruing	49	261	263
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,799	4,880	3,152
Total Current	685,607	635,074	608,940
Total	698,406	639,954	612,092
Total 90 Days Past Due Still Accruing		24	111
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	230	479	353
Total Current	99,002	105,215	86,365
Total	99,232	105,694	86,718
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	22,987	14,577	18,550
Total Current	2,065,015	1,808,277	1,756,089
Total	2,088,002	1,822,854	1,774,639
Total 90 Days Past Due Still Accruing	21	207	102
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,177	2,952	2,331
Total Current	379,000	357,734	352,650
Total	381,177	360,686	354,981
Total 90 Days Past Due Still Accruing	28	30	50
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	32,772	19,372	20,166
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	11,554	4,611	2,726
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	169	437	294
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,392	12,002	15,404
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,657	2,322	1,742
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,805	1,339	1,391
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,043	94	110
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	28	42	59
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,267	707	930
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	467	496	292
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,616	2,177	2,829
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	202	175	316
Greater than 90 Days [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	33
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,328	1,868	2,216
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 53	$ 134	$ 297