Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 3,021	$ 3,416
Impaired Financing Receivable, Interest Income, Accrual Method	2	2
Related allowance	204		$ 0
Recorded investment	4,178		2,519
Unpaid principal balance	4,680		2,993
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	74	439
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	74		73
Unpaid principal balance	74		73
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	2,090	1,981
Impaired Financing Receivable, Interest Income, Accrual Method	2	1
Recorded investment	2,000		2,136
Unpaid principal balance	2,468		2,585
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	229	982
Impaired Financing Receivable, Interest Income, Accrual Method	0	1
Recorded investment	245		298
Unpaid principal balance	261		320
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	12	14
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	12		12
Unpaid principal balance	15		15
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	2,405	3,416
Impaired Financing Receivable, Interest Income, Accrual Method	2	2
Recorded investment	2,331		2,519
Unpaid principal balance	2,818		2,993
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	616	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	204		0
Recorded investment	1,847		0
Unpaid principal balance	1,862		0
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Acquired Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	616	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	204		0
Recorded investment	1,847		0
Unpaid principal balance	1,862		0
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	37,262	24,773
Impaired Financing Receivable, Interest Income, Accrual Method	126	131
Related allowance	9,595		2,432
Recorded investment	54,962		29,698
Unpaid principal balance	65,626		39,122
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	7,838	3,068
Impaired Financing Receivable, Interest Income, Accrual Method	18	3
Related allowance	2,727		1,580
Recorded investment	7,469		8,290
Unpaid principal balance	14,780		10,032
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	925	2,707
Impaired Financing Receivable, Interest Income, Accrual Method	4	13
Recorded investment	958		1,848
Unpaid principal balance	1,143		6,997
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Recorded investment	0		0
Unpaid principal balance	0		0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	10,529	10,798
Impaired Financing Receivable, Interest Income, Accrual Method	78	72
Recorded investment	9,909		10,372
Unpaid principal balance	12,056		12,437
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	4,086	3,994
Impaired Financing Receivable, Interest Income, Accrual Method	22	36
Recorded investment	4,447		3,015
Unpaid principal balance	4,965		3,210
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	445	324
Impaired Financing Receivable, Interest Income, Accrual Method	3	1
Recorded investment	473		406
Unpaid principal balance	840		640
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	15,985	17,823
Impaired Financing Receivable, Interest Income, Accrual Method	107	122
Recorded investment	15,787		15,641
Unpaid principal balance	19,004		23,284
Originated Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	325	604
Impaired Financing Receivable, Interest Income, Accrual Method	0	5
Related allowance	0		1
Recorded investment	255		474
Unpaid principal balance	358		498
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	13,114	3,278
Impaired Financing Receivable, Interest Income, Accrual Method	1	1
Related allowance	6,868		851
Recorded investment	31,451		5,293
Unpaid principal balance	31,484		5,308
Originated Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0
Related allowance	0		0
Recorded investment	0		0
Unpaid principal balance	0		0
Originated Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	21,277	6,950
Impaired Financing Receivable, Interest Income, Accrual Method	19	$ 9
Related allowance	9,595		2,432
Recorded investment	39,175		14,057
Unpaid principal balance	$ 46,622		$ 15,838