Loans Held for Investment and Allowance for Credit Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 4,639,389	$ 4,194,969	$ 3,989,160
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	869,450	856,326	826,886
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	99,582	103,640	91,336
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,249,249	2,823,372	2,684,207
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	421,108	411,631	386,731
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	4,449,041	4,054,314	3,870,727
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	831,385	825,775	787,651
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	92,447	101,614	87,151
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,106,100	2,717,227	2,611,139
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	419,109	409,698	384,786
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	87,099	63,371	45,275
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	25,390	20,971	23,182
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	5,270	64	20
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	56,114	42,036	21,827
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	325	300	246
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	103,249	77,284	73,158
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	12,675	9,580	16,053
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,865	1,962	4,165
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	87,035	64,109	51,241
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 1,674	$ 1,633	$ 1,699