Loans Held for Investment and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	$ 53,588,000	$ 35,235,000	$ 53,588,000	$ 35,235,000	$ 33,949,000
Recorded Investment With No Allowance	21,638,000	11,707,000	21,638,000	11,707,000	8,369,000
Recorded Investment With Allowance	17,682,000	14,701,000	17,682,000	14,701,000	16,213,000
Total Recorded Investment	39,320,000	26,408,000	39,320,000	26,408,000	24,582,000
Related Allowance	3,046,000	3,866,000	3,046,000	3,866,000	3,228,000
Average Recorded Investment	41,410,000	27,165,000	42,681,000	28,308,000	27,307,000
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	6,013,000	9,340,000	6,013,000	9,340,000	4,511,000
Recorded Investment With No Allowance	739,000	6,212,000	739,000	6,212,000	630,000
Recorded Investment With Allowance	3,879,000	1,977,000	3,879,000	1,977,000	2,463,000
Total Recorded Investment	4,618,000	8,189,000	4,618,000	8,189,000	3,093,000
Related Allowance	1,250,000	1,022,000	1,250,000	1,022,000	1,042,000
Average Recorded Investment	23,572,000	8,562,000	24,270,000	8,699,000	3,488,000
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	1,287,000	1,108,000	1,287,000	1,108,000	1,603,000
Recorded Investment With No Allowance	529,000	434,000	529,000	434,000	658,000
Recorded Investment With Allowance	527,000	613,000	527,000	613,000	718,000
Total Recorded Investment	1,056,000	1,047,000	1,056,000	1,047,000	1,376,000
Related Allowance	83,000	119,000	83,000	119,000	235,000
Average Recorded Investment	179,000	1,071,000	219,000	1,090,000	1,644,000
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	45,703,000	24,262,000	45,703,000	24,262,000	27,366,000
Recorded Investment With No Allowance	20,370,000	5,034,000	20,370,000	5,034,000	7,081,000
Recorded Investment With Allowance	12,800,000	11,753,000	12,800,000	11,753,000	12,706,000
Total Recorded Investment	33,170,000	16,787,000	33,170,000	16,787,000	19,787,000
Related Allowance	1,711,000	2,515,000	1,711,000	2,515,000	1,950,000
Average Recorded Investment	17,107,000	17,105,000	17,586,000	18,054,000	21,726,000
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	585,000	525,000	585,000	525,000	469,000
Recorded Investment With No Allowance	0	27,000	0	27,000	0
Recorded Investment With Allowance	476,000	358,000	476,000	358,000	326,000
Total Recorded Investment	476,000	385,000	476,000	385,000	326,000
Related Allowance	2,000	210,000	2,000	210,000	1,000
Average Recorded Investment	$ 552,000	$ 427,000	$ 606,000	$ 465,000	$ 449,000