Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 4,040,952	$ 3,953,636	$ 3,831,480
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	813,887	844,953	775,010
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	97,535	96,677	92,583
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,730,585	2,639,346	2,567,878
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	398,945	372,660	396,009
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,914,912	3,827,337	3,717,754
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	767,275	804,584	749,387
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	94,481	92,864	87,768
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,656,139	2,559,379	2,487,399
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	397,017	370,510	393,200
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	51,490	50,379	34,557
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	31,124	23,392	6,439
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	58	46	1,568
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	20,044	26,626	26,182
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	264	315	368
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	74,550	75,920	79,169
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	15,488	16,977	19,184
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,996	3,767	3,247
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	54,402	53,341	54,297
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 1,664	$ 1,835	$ 2,441