Loans Held for Investment and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018		Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	$ 35,235		$ 34,412		$ 35,235		$ 34,412		$ 36,614
Recorded Investment With No Allowance	11,707	[1]	7,052	[2]	11,707	[1]	7,052	[2]	13,741	[3]
Recorded Investment With Allowance	14,701		19,633		14,701		19,633		13,793
Total Recorded Investment	26,408		26,685		26,408		26,685		27,534
Related Allowance	3,866		4,823		3,866		4,823		4,069
Average Recorded Investment	27,165		29,026		28,308		29,836		25,784
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	9,340		9,432		9,340		9,432		10,808
Recorded Investment With No Allowance	6,212	[1]	3,264	[2]	6,212	[1]	3,264	[2]	6,728	[3]
Recorded Investment With Allowance	1,977		4,316		1,977		4,316		2,606
Total Recorded Investment	8,189		7,580		8,189		7,580		9,334
Related Allowance	1,022		1,379		1,022		1,379		1,133
Average Recorded Investment	8,562		8,226		8,699		8,485		7,986
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	1,108		1,278		1,108		1,278		799
Recorded Investment With No Allowance	434	[1]	50	[2]	434	[1]	50	[2]	213	[3]
Recorded Investment With Allowance	613		1,209		613		1,209		546
Total Recorded Investment	1,047		1,259		1,047		1,259		759
Related Allowance	119		368		119		368		170
Average Recorded Investment	1,071		1,589		1,090		1,670		842
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	24,262		22,358		24,262		22,358		24,072
Recorded Investment With No Allowance	5,034	[1]	3,569	[2]	5,034	[1]	3,569	[2]	6,699	[3]
Recorded Investment With Allowance	11,753		13,146		11,753		13,146		10,015
Total Recorded Investment	16,787		16,715		16,787		16,715		16,714
Related Allowance	2,515		2,645		2,515		2,645		2,409
Average Recorded Investment	17,105		18,019		18,054		18,441		16,042
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	525		1,344		525		1,344		935
Recorded Investment With No Allowance	27	[1]	169	[2]	27	[1]	169	[2]	101	[3]
Recorded Investment With Allowance	358		962		358		962		626
Total Recorded Investment	385		1,131		385		1,131		727
Related Allowance	210		431		210		431		357
Average Recorded Investment	$ 427		$ 1,192		$ 465		$ 1,240		$ 914

[1] Includes $464,000 of purchased credit impaired loans.

[2] Includes $2,813,000 of purchased credit impaired loans.

[3] Includes $827,000 of purchased credit impaired loans.