Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,953,636	$ 3,485,569
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	844,953	684,099
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	96,677	94,543
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,639,346	2,302,998
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	372,660	403,929
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,827,337	3,368,359
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	804,584	649,166
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	92,864	90,457
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,559,379	2,227,302
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	370,510	401,434
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	50,379	37,079
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	23,392	6,282
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	46	1,527
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	26,626	29,089
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	315	181
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	75,920	80,131
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	16,977	28,651
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,767	2,559
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	53,341	46,607
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 1,835	$ 2,314