Loans Held-for-Investment and Allowance for Loan Losses - Schedule of Company's Internal Ratings of its Loans Held-for-Investment, Including The Year of Origination, by Portfolio Segments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
2021	$ 754,000	$ 2,418,000
2020	2,070,000	832,000
2019	729,000	571,000
2018	510,000	350,000
2017	321,000	279,000
Prior	837,000	619,000
Revolving Loans Amort Cost Basis	102,000	102,000
Total	5,323,000	5,171,000
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	306,000	895,000
2020	656,000	107,000
2019	86,000	74,000
2018	64,000	29,000
2017	26,000	10,000
Prior	41,000	16,000
Total	1,179,000	1,131,000
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,000	28,000
2020	23,000	19,000
2019	16,000	29,000
2018	23,000	19,000
2017	19,000	14,000
Prior	94,000	72,000
Total	177,000	181,000
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	16,000	63,000
2020	47,000	19,000
2019	16,000	9,000
2018	7,000	3,000
2017	3,000	1,000
Prior	1,000
Total	90,000	95,000
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	102,000	377,000
2020	352,000	102,000
2019	67,000	36,000
2018	35,000	22,000
2017	17,000	7,000
Prior	15,000	10,000
Total	588,000	554,000
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	20,000	64,000
2020	61,000	23,000
2019	22,000	18,000
2018	18,000	12,000
2017	10,000	11,000
Prior	31,000	24,000
Total	162,000	152,000
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	45,000	198,000
2020	199,000	119,000
2019	120,000	94,000
2018	87,000	52,000
2017	51,000	56,000
Prior	148,000	99,000
Total	650,000	618,000
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	75,000	186,000
2020	176,000	141,000
2019	134,000	127,000
2018	114,000	83,000
2017	80,000	67,000
Prior	181,000	143,000
Total	760,000	747,000
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	110,000	381,000
2020	354,000	176,000
2019	160,000	138,000
2018	124,000	105,000
2017	96,000	103,000
Prior	316,000	250,000
Revolving Loans Amort Cost Basis	95,000	95,000
Total	1,255,000	1,248,000
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	61,000	178,000
2020	162,000	105,000
2019	91,000	39,000
2018	32,000	21,000
2017	16,000	9,000
Prior	8,000	2,000
Total	370,000	354,000
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	17,000	48,000
2020	40,000	21,000
2019	17,000	7,000
2018	6,000	4,000
2017	3,000	1,000
Prior	2,000	3,000
Revolving Loans Amort Cost Basis	7,000	7,000
Total	92,000	91,000
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	747,000	2,356,000
2020	2,024,000	804,000
2019	694,000	540,000
2018	482,000	320,000
2017	291,000	273,000
Prior	794,000	580,000
Revolving Loans Amort Cost Basis	101,000	100,000
Total	5,133,000	4,973,000
Pass [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	303,000	874,000
2020	647,000	101,000
2019	84,000	70,000
2018	61,000	28,000
2017	24,000	10,000
Prior	41,000	16,000
Total	1,160,000	1,099,000
Pass [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,000	26,000
2020	21,000	19,000
2019	16,000	29,000
2018	23,000	14,000
2017	13,000	13,000
Prior	92,000	71,000
Total	167,000	172,000
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	15,000	57,000
2020	42,000	19,000
2019	16,000	9,000
2018	7,000	3,000
2017	2,000	1,000
Prior	1,000
Total	83,000	89,000
Pass [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	101,000	371
2020	347,000	97,000
2019	62,000	36,000
2018	35,000	19,000
2017	17,000	7,000
Prior	13,000	9,000
Total	575,000	539,000
Pass [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	20,000	57,000
2020	54,000	22,000
2019	21,000	18,000
2018	17,000	11,000
2017	10,000	11,000
Prior	30,000	23,000
Total	152,000	142,000
Pass [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	45,000	197,000
2020	198,000	117,000
2019	105,000	93,000
2018	86,000	44,000
2017	41,000	55,000
Prior	136,000	88,000
Total	611,000	594,000
Pass [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	74,000	176,000
2020	168,000	132,000
2019	126,000	105,000
2018	96,000	75,000
2017	72,000	65,000
Prior	169,000	132,000
Total	705,000	685,000
Pass [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	109,000	373,000
2020	346,000	172,000
2019	157,000	134,000
2018	119,000	101,000
2017	93,000	101,000
Prior	302,000	237,000
Revolving Loans Amort Cost Basis	94,000	93,000
Total	1,220,000	1,211,000
Pass [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	61,000	177,000
2020	161,000	104,000
2019	90,000	39,000
2018	32,000	21,000
2017	16,000	9,000
Prior	8,000	2,000
Total	368,000	352,000
Pass [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	17,000	48,000
2020	40,000	21,000
2019	17,000	7,000
2018	6,000	4,000
2017	3,000	1,000
Prior	2,000	2,000
Revolving Loans Amort Cost Basis	7,000	7,000
Total	92,000	90,000
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	3,000	20,000
2020	12,000	12,000
2019	23,000	4,000
2018	2,000	14,000
2017	14,000	3,000
Prior	8,000	4,000
Total	62,000	57,000
Special Mention [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,000	9,000
2020	3,000	2,000
2019	1,000
2018		1,000
2017	1,000
Total	7,000	12,000
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,000
Total	1,000
Special Mention [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		2,000
2020	2,000	4,000
2019	4,000
Prior	1,000
Total	7,000	6,000
Special Mention [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		1,000
2020	1,000
2019	13,000	1,000
2018	1,000	8,000
2017	8,000	1,000
Prior	4,000
Total	27,000	11,000
Special Mention [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		5,000
2020	3,000	5,000
2019	4,000	2,000
2018		4,000
2017	4,000	1,000
Prior		1,000
Total	11,000	18,000
Special Mention [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		3,000
2020	3,000	1,000
2019	1,000	1,000
2018	1,000	1,000
2017	1,000	1,000
Prior	3,000	3,000
Total	9,000	10,000
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	4,000	42,000
2020	34,000	16,000
2019	12,000	27,000
2018	26,000	16,000
2017	16,000	3,000
Prior	35,000	35,000
Revolving Loans Amort Cost Basis		2,000
Total	128,000	141,000
Substandard [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,000	12,000
2020	6,000	4,000
2019	1,000	4,000
2018	3,000
2017	1,000
Total	12,000	20,000
Substandard [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		2,000
2020	2,000
2018		5,000
2017	6,000	1,000
Prior	2,000	1,000
Total	10,000	9,000
Substandard [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		6,000
2020	5,000
2017	1,000
Total	6,000	6,000
Substandard [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,000	4,000
2020	3,000	1,000
2019	1,000
2018		3,000
Prior	1,000	1,000
Total	6,000	9,000
Substandard [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		7,000
2020	7,000	1,000
2019	1,000
2018	1,000	1,000
Prior	1,000	1,000
Total	10,000	10,000
Substandard [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020		2,000
2019	2,000
2017	2,000
Prior	8,000	11,000
Total	12,000	13,000
Substandard [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,000	5,000
2020	5,000	4,000
2019	4,000	20,000
2018	18,000	4,000
2017	4,000	1,000
Prior	12,000	10,000
Total	44,000	44,000
Substandard [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,000	5,000
2020	5,000	3,000
2019	2,000	3,000
2018	4,000	3,000
2017	2,000	1,000
Prior	11,000	10,000
Revolving Loans Amort Cost Basis		2,000
Total	26,000	27,000
Substandard [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021		1,000
2020	1,000	1,000
2019	1,000
Total	$ 2,000	2,000
Substandard [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior		1,000
Total		$ 1,000