Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 33,312	$ 27,647	$ 38,193
Total Current	3,304,481	3,289,403	3,228,624
Total	3,337,793	3,317,050	3,266,817
Total 90 Days Past Due Still Accruing	107	341	49
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,498	7,775	12,799
Total Current	651,083	688,388	685,607
Total	663,581	696,163	698,406
Total 90 Days Past Due Still Accruing	61	54
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	586	431	230
Total Current	84,130	101,920	99,002
Total	84,716	102,351	99,232
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	18,316	17,347	22,987
Total Current	2,172,944	2,118,886	2,065,015
Total	2,191,260	2,136,233	2,088,002
Total 90 Days Past Due Still Accruing	34	217	21
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,912	2,094	2,177
Total Current	396,324	380,209	379,000
Total	398,236	382,303	381,177
Total 90 Days Past Due Still Accruing	12	70	28
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	20,092	17,339	32,772
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,707	3,099	11,554
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	523	348	169
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13,444	12,247	19,392
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,418	1,645	1,657
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,714	6,144	2,805
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	841	3,652	1,043
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	63	83	28
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,496	2,226	1,267
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	314	183	467
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,506	4,164	2,616
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,950	1,024	202
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	3,376	2,874	2,328
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 180	$ 266	$ 53