Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,831,480	$ 3,485,569	$ 3,439,352
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	775,010	684,099	668,049
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	92,583	94,543	77,342
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,567,878	2,302,998	2,271,100
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	396,009	403,929	422,861
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,717,754	3,368,359	3,321,744
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	749,387	649,166	627,812
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	87,768	90,457	73,727
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,487,399	2,227,302	2,200,067
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	393,200	401,434	420,138
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	34,557	37,079	27,999
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	6,439	6,282	5,965
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,568	1,527	859
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	26,182	29,089	20,978
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	368	181	197
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	79,169	80,131	89,609
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	19,184	28,651	34,272
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,247	2,559	2,756
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	54,297	46,607	50,055
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2,441	$ 2,314	$ 2,526