Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 6,575,840	$ 5,714,373
Originated Loans [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	6,394,652	5,624,757
Originated Loans [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	107,420	43,393
Originated Loans [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	73,768	46,223
Originated Loans [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	181,188	89,616
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,711,614	1,212,026
Originated Loans [Member] | Commercial, financial, agricultural and other [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,649,458	1,171,363
Originated Loans [Member] | Commercial, financial, agricultural and other [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	37,033	29,359
Originated Loans [Member] | Commercial, financial, agricultural and other [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	25,123	11,304
Originated Loans [Member] | Commercial, financial, agricultural and other [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Commercial, financial, agricultural and other [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	62,156	40,663
Originated Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	450,589	442,777
Originated Loans [Member] | Real estate construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	450,513	442,751
Originated Loans [Member] | Real estate construction [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	22	26
Originated Loans [Member] | Real estate construction [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	54	0
Originated Loans [Member] | Real estate construction [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Real estate construction [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	76	26
Originated Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,534,949	1,415,808
Originated Loans [Member] | Residential real estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,527,225	1,406,845
Originated Loans [Member] | Residential real estate [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	915	475
Originated Loans [Member] | Residential real estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	6,809	8,488
Originated Loans [Member] | Residential real estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Residential real estate [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	7,724	8,963
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	2,088,217	1,958,346
Originated Loans [Member] | Commercial Real Estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,977,234	1,918,690
Originated Loans [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	69,450	13,533
Originated Loans [Member] | Commercial Real Estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	41,533	26,123
Originated Loans [Member] | Commercial Real Estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Commercial Real Estate [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	110,983	39,656
Originated Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	790,471	685,416
Originated Loans [Member] | Loans to individuals [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	790,222	685,108
Originated Loans [Member] | Loans to individuals [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Loans to individuals [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	249	308
Originated Loans [Member] | Loans to individuals [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Originated Loans [Member] | Loans to individuals [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	249	308
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	373,876	474,775
Acquired Loans [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	366,107	463,784
Acquired Loans [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,347	5,183
Acquired Loans [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	6,422	5,808
Acquired Loans [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	7,769	10,991
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	25,122	29,827
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	23,405	27,696
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	196	2,009
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,521	122
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,717	2,131
Acquired Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	2,400	6,262
Acquired Loans [Member] | Real estate construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,588	5,697
Acquired Loans [Member] | Real estate construction [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	504	565
Acquired Loans [Member] | Real estate construction [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	308	0
Acquired Loans [Member] | Real estate construction [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Real estate construction [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	812	565
Acquired Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	209,071	265,554
Acquired Loans [Member] | Residential real estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	207,233	262,630
Acquired Loans [Member] | Residential real estate [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	511	537
Acquired Loans [Member] | Residential real estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,327	2,387
Acquired Loans [Member] | Residential real estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Residential real estate [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,838	2,924
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	127,094	159,173
Acquired Loans [Member] | Commercial Real Estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	123,703	153,814
Acquired Loans [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	136	2,072
Acquired Loans [Member] | Commercial Real Estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	3,255	3,287
Acquired Loans [Member] | Commercial Real Estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Commercial Real Estate [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	3,391	5,359
Acquired Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	10,189	13,959
Acquired Loans [Member] | Loans to individuals [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	10,178	13,947
Acquired Loans [Member] | Loans to individuals [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Loans to individuals [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	11	12
Acquired Loans [Member] | Loans to individuals [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Acquired Loans [Member] | Loans to individuals [Member] | Criticized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 11	$ 12