Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 4,907,961	$ 4,879,347	$ 4,798,755
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,148,460		1,190,384
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	240,122		256,856
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,217,398		1,212,962
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,761,101		1,552,904
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	540,880		$ 585,649
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	1,173	1,302
Financing Receivable, Recorded Investment, Current	93,513	99,574
Total	94,686	100,876
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	477	486
Financing Receivable, Recorded Investment, Current	7,281	7,913
Total	7,758	8,399
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	139	1,781
Total	139	1,781
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	525	643
Financing Receivable, Recorded Investment, Current	59,637	62,698
Total	60,162	63,341
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	162	162
Financing Receivable, Recorded Investment, Current	24,576	25,005
Total	24,738	25,167
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	9	11
Financing Receivable, Recorded Investment, Current	1,880	2,177
Total	1,889	2,188
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	79	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]
Total past due and nonaccrual	0	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]
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	73	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	6	1
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	105	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]
Total past due and nonaccrual	0	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]
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	103	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]
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	2	7
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	529	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]
Total past due and nonaccrual	477	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	51	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]
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	1	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	460	587
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	298	422
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	162	162
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	3
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	43,366	40,497
Financing Receivable, Recorded Investment, Current	4,769,909	4,737,974
Total	4,813,275	4,778,471
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	25,425	20,554
Financing Receivable, Recorded Investment, Current	1,115,277	1,110,594
Total	1,140,702	1,131,148
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	183
Financing Receivable, Recorded Investment, Current	239,983	217,657
Total	239,983	217,840
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	10,767	12,009
Financing Receivable, Recorded Investment, Current	1,146,469	1,153,842
Total	1,157,236	1,165,851
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	4,651	4,092
Financing Receivable, Recorded Investment, Current	1,731,712	1,712,951
Total	1,736,363	1,717,043
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	2,523	3,659
Financing Receivable, Recorded Investment, Current	536,468	542,930
Total	538,991	546,589
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	7,690	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]
Total past due and nonaccrual	2,973	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]
Total past due and nonaccrual	0	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]
Total past due and nonaccrual	2,979	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]
Total past due and nonaccrual	665	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]
Total past due and nonaccrual	1,073	1,640
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	2,275	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]
Total past due and nonaccrual	603	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]
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	1,187	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]
Total past due and nonaccrual	0	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]
Total past due and nonaccrual	485	776
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	1,582	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]
Total past due and nonaccrual	139	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]
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	617	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]
Total past due and nonaccrual	87	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]
Total past due and nonaccrual	739	970
Originated Loans [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	31,819	27,436
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	21,710	17,928
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,984	5,792
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,899	3,443
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	$ 226	$ 273