Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,407,376	$ 4,879,347	$ 4,683,750
Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,163,383	1,139,547
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	248,868	219,621
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,426,370	1,229,192
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,019,096	1,742,210
Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	549,659	548,777
Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,163,383	1,139,547	1,150,906
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	248,868	219,621	220,736
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,426,370	1,229,192	1,224,465
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,019,096	1,742,210	1,479,000
Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	549,659	548,777	$ 608,643
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	41,362	40,497
Current	4,965,999	4,737,974
Loans	5,007,361	4,778,471
Originated Loans [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	19,220	20,554
Current	1,103,521	1,110,594
Loans	1,122,741	1,131,148
Originated Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	199	183
Current	242,706	217,657
Loans	242,905	217,840
Originated Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	12,944	12,009
Current	1,193,175	1,153,842
Loans	1,206,119	1,165,851
Originated Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,472	4,092
Current	1,886,713	1,712,951
Loans	1,892,185	1,717,043
Originated Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,527	3,659
Current	539,884	542,930
Loans	543,411	546,589
Originated Loans [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,122,741	1,131,148
Originated Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	242,905	217,840
Originated Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,206,119	1,165,851
Originated Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,892,185	1,717,043
Originated Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	543,411	546,589
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	9,292	8,601
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	378	2,380
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	199	183
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,618	4,133
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,198	265
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,899	1,640
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,883	2,363
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	61	171
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,025	1,089
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	28	327
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	769	776
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,745	2,097
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	40	75
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,076	995
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	6	57
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	623	970
Originated Loans [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	28,442	27,436
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	18,741	17,928
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	6,225	5,792
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,240	3,443
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	236	273
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,570	1,302
Current	397,445	99,574
Loans	400,015	100,876
Acquired Loans [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	449	486
Current	40,193	7,913
Loans	40,642	8,399
Acquired Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Current	5,963	1,781
Loans	5,963	1,781
Acquired Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	945	643
Current	219,306	62,698
Loans	220,251	63,341
Acquired Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,077	162
Current	125,834	25,005
Loans	126,911	25,167
Acquired Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	99	11
Current	6,149	2,177
Loans	6,248	2,188
Acquired Loans [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	40,642	8,399
Acquired Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5,963	1,781
Acquired Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	220,251	63,341
Acquired Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	126,911	25,167
Acquired Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,248	2,188
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	190	635
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	6	486
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	148	148
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	36	1
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	36	46
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	7	0
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	9	39
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	20	7
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	109	34
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	83	34
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	26	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,235	587
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	436	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	705	422
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,077	162
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 17	$ 3