Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 5,381,305	$ 5,407,376	$ 4,907,961
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,131,594		1,148,460
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	246,961		240,122
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,434,623		1,217,398
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,027,072		1,761,101
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	541,055		$ 540,880
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	2,529	2,570
Financing Receivable, Recorded Investment, Current	381,661	397,445
Total	384,190	400,015
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	448	449
Financing Receivable, Recorded Investment, Current	37,954	40,193
Total	38,402	40,642
Acquired Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	0	0
Financing Receivable, Recorded Investment, Current	1,120	5,963
Total	1,120	5,963
Acquired Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	968	945
Financing Receivable, Recorded Investment, Current	212,666	219,306
Total	213,634	220,251
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	1,000	1,077
Financing Receivable, Recorded Investment, Current	124,463	125,834
Total	125,463	126,911
Acquired Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	113	99
Financing Receivable, Recorded Investment, Current	5,458	6,149
Total	5,571	6,248
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	196	190
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]
Total past due and nonaccrual	11	6
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	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]
Total past due and nonaccrual	119	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]
Total past due and nonaccrual	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]
Total past due and nonaccrual	66	36
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	121	36
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]
Total past due and nonaccrual	0	7
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	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]
Total past due and nonaccrual	110	9
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	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]
Total past due and nonaccrual	11	20
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	66	109
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]
Total past due and nonaccrual	26	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]
Total past due and nonaccrual	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]
Total past due and nonaccrual	21	83
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]
Total past due and nonaccrual	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]
Total past due and nonaccrual	19	26
Acquired Loans [Member] | Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,146	2,235
Acquired Loans [Member] | Non Accrual Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	411	436
Acquired Loans [Member] | Non Accrual Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Non Accrual Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	718	705
Acquired Loans [Member] | Non Accrual Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,000	1,077
Acquired Loans [Member] | Non Accrual Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	17	17
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	48,690	41,362
Financing Receivable, Recorded Investment, Current	4,948,425	4,965,999
Total	4,997,115	5,007,361
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	26,652	19,220
Financing Receivable, Recorded Investment, Current	1,066,540	1,103,521
Total	1,093,192	1,122,741
Originated Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	0	199
Financing Receivable, Recorded Investment, Current	245,841	242,706
Total	245,841	242,905
Originated Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	10,681	12,944
Financing Receivable, Recorded Investment, Current	1,210,308	1,193,175
Total	1,220,989	1,206,119
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	8,513	5,472
Financing Receivable, Recorded Investment, Current	1,893,096	1,886,713
Total	1,901,609	1,892,185
Originated Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	2,844	3,527
Financing Receivable, Recorded Investment, Current	532,640	539,884
Total	535,484	543,411
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	8,132	9,292
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]
Total past due and nonaccrual	2,163	378
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	199
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,849	4,618
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,751	2,198
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,369	1,899
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,265	1,883
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]
Total past due and nonaccrual	23	61
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	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]
Total past due and nonaccrual	996	1,025
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	783	28
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	463	769
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,889	1,745
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]
Total past due and nonaccrual	168	40
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]
Total past due and nonaccrual	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]
Total past due and nonaccrual	649	1,076
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]
Total past due and nonaccrual	304	6
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]
Total past due and nonaccrual	768	623
Originated Loans [Member] | Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	36,404	28,442
Originated Loans [Member] | Non Accrual Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	24,298	18,741
Originated Loans [Member] | Non Accrual Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Originated Loans [Member] | Non Accrual Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	6,187	6,225
Originated Loans [Member] | Non Accrual Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,675	3,240
Originated Loans [Member] | Non Accrual Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	$ 244	$ 236