Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 5,253,067	$ 4,194,969	$ 4,040,952
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,509,454	856,326	813,887
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	97,448	103,640	97,535
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,235,208	2,823,372	2,730,585
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	410,957	411,631	398,945
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	5,052,170	4,054,314	3,914,912
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,465,595	825,775	767,275
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	90,824	101,614	94,481
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,086,894	2,717,227	2,656,139
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	408,857	409,698	397,017
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	63,489	63,371	51,490
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	8,950	20,971	31,124
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	4,790	64	58
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	49,493	42,036	20,044
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	256	300	264
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	137,408	77,284	74,550
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	34,909	9,580	15,488
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,834	1,962	2,996
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	98,821	64,109	54,402
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 1,844	$ 1,633	$ 1,664