Loans Held for Investment and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 21,373	$ 19,304	$ 22,808
Total Current	5,231,694	4,175,665	4,018,144
Total Loans	5,253,067	4,194,969	4,040,952
Total 90 Days Past Due Still Accruing	92	153	300
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,166	4,536	3,633
Total Current	1,504,288	851,790	810,254
Total Loans	1,509,454	856,326	813,887
Total 90 Days Past Due Still Accruing	0	112	58
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,095	627	508
Total Current	96,353	103,013	97,027
Total Loans	97,448	103,640	97,535
Total 90 Days Past Due Still Accruing			30
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,514	13,373	17,707
Total Current	3,220,694	2,809,999	2,712,878
Total Loans	3,235,208	2,823,372	2,730,585
Total 90 Days Past Due Still Accruing			180
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	598	768	960
Total Current	410,359	410,863	397,985
Total Loans	410,957	411,631	398,945
Total 90 Days Past Due Still Accruing	92	41	32
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,731	16,902	19,937
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,563	3,257	2,671
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,080	183	315
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13,678	12,890	16,130
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	410	572	821
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	682	1,040	1,326
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	474	557	352
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15	44	163
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	110	288	712
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	83	151	99
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	960	1,362	1,545
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	129	722	610
Greater than 90 Days [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	400	30
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	726	195	865
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 105	$ 45	$ 40