Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 46,836	$ 36,705
Total past due and nonaccrual	57,032	49,636
Current	4,741,723	4,634,114
Total	4,798,755	4,683,750	$ 4,437,601
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	34,851	23,653
Total past due and nonaccrual	35,350	24,195
Current	1,155,034	1,126,711
Total	1,190,384	1,150,906
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	0	28
Total past due and nonaccrual	86	308
Current	256,770	220,428
Total	256,856	220,736
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	6,642	6,500
Total past due and nonaccrual	11,794	13,045
Current	1,201,168	1,211,420
Total	1,212,962	1,224,465
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	4,963	6,223
Total past due and nonaccrual	6,233	7,069
Current	1,546,671	1,471,931
Total	1,552,904	1,479,000
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	380	301
Total past due and nonaccrual	3,569	5,019
Current	582,080	603,624
Total	585,649	608,643
Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	6,918	8,598
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	277	364
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	280
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	3,639	4,175
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	1,270	781
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	1,732	2,998
Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,948	1,878
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	92	49
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
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	1,308	1,055
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
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	548	774
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,330	2,455
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	130	129
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	86	0
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	205	1,315
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	65
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	$ 909	$ 946