Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,867,040	$ 3,485,569	$ 3,472,227
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	773,924	684,099	674,947
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	93,953	94,543	83,005
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,614,929	2,302,998	2,297,556
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	384,234	403,929	416,719
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,749,517	3,368,359	3,350,933
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	747,758	649,166	632,693
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	89,314	90,457	79,227
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,530,673	2,227,302	2,224,970
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	381,772	401,434	414,043
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	36,450	37,079	35,237
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	8,817	6,282	7,997
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	68	1,527	841
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	27,241	29,089	26,231
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	324	181	168
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	81,073	80,131	86,057
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	17,349	28,651	34,257
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	4,571	2,559	2,937
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	57,015	46,607	46,355
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2,138	$ 2,314	$ 2,508