Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 50,725	$ 57,452	$ 54,225	$ 67,187
Charge-offs	(2,880)	(5,583)	(14,629)	(32,195)
Recoveries	866	374	2,567	1,954
Provision for Loan, Lease, and Other Losses	2,073	2,714	8,621	18,011
Ending Balance	50,784	54,957	50,784	54,957
Ending balance: individually evaluated for impaired	4,632	8,239	4,632	8,239
Ending balance: collectively evaluated for impaired	46,152	46,718	46,152	46,718
Loans and Leases Receivable, Gross	4,411,481		4,411,481		4,283,833
Total	4,411,481	4,240,004	4,411,481	4,240,004	4,283,833
Ending balance: individually evaluated for impaired	37,716	64,639	37,716	64,639
Ending balance: collectively evaluated for impaired	4,373,765	4,175,365	4,373,765	4,175,365
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	21,956	16,575	22,663	19,852
Charge-offs	(498)	(3,081)	(8,357)	(17,302)
Recoveries	204	81	625	345
Provision for Loan, Lease, and Other Losses	(1,958)	1,358	4,773	12,038
Ending Balance	19,704	14,933	19,704	14,933
Ending balance: individually evaluated for impaired	4,271	4,889	4,271	4,889
Ending balance: collectively evaluated for impaired	15,433	10,044	15,433	10,044
Loans and Leases Receivable, Gross	1,071,531		1,071,531		1,021,056
Total		1,008,316		1,008,316
Ending balance: individually evaluated for impaired	23,773	25,620	23,773	25,620
Ending balance: collectively evaluated for impaired	1,047,758	982,696	1,047,758	982,696
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,899	8,246	6,600	8,928
Charge-offs	0	(18)	(296)	(773)
Recoveries	132	81	469	140
Provision for Loan, Lease, and Other Losses	2,073	(1,879)	1,331	(1,865)
Ending Balance	8,104	6,430	8,104	6,430
Ending balance: individually evaluated for impaired	0	165	0	165
Ending balance: collectively evaluated for impaired	8,104	6,265	8,104	6,265
Loans and Leases Receivable, Gross	115,788		115,788		93,289
Total		74,025		74,025
Ending balance: individually evaluated for impaired	199	4,028	199	4,028
Ending balance: collectively evaluated for impaired	115,589	69,997	115,589	69,997
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6,125	6,439	7,727	5,908
Charge-offs	(551)	(757)	(2,286)	(1,400)
Recoveries	97	71	420	883
Provision for Loan, Lease, and Other Losses	(217)	84	(407)	446
Ending Balance	5,454	5,837	5,454	5,837
Ending balance: individually evaluated for impaired	332	1,377	332	1,377
Ending balance: collectively evaluated for impaired	5,122	4,460	5,122	4,460
Loans and Leases Receivable, Gross	1,234,842		1,234,842		1,262,718
Total		1,280,990		1,280,990
Ending balance: individually evaluated for impaired	6,854	9,794	6,854	9,794
Ending balance: collectively evaluated for impaired	1,227,988	1,271,196	1,227,988	1,271,196
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	11,661	16,128	11,778	22,441
Charge-offs	(812)	(813)	(1,109)	(10,051)
Recoveries	177	13	432	121
Provision for Loan, Lease, and Other Losses	1,528	1,501	1,453	4,318
Ending Balance	12,554	16,829	12,554	16,829
Ending balance: individually evaluated for impaired	29	1,808	29	1,808
Ending balance: collectively evaluated for impaired	12,525	15,021	12,525	15,021
Loans and Leases Receivable, Gross	1,345,302		1,345,302		1,296,472
Total		1,272,366		1,272,366
Ending balance: individually evaluated for impaired	6,890	25,197	6,890	25,197
Ending balance: collectively evaluated for impaired	1,338,412	1,247,169	1,338,412	1,247,169
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,084	4,142	5,457	4,132
Charge-offs	(1,019)	(914)	(2,581)	(2,669)
Recoveries	256	128	621	465
Provision for Loan, Lease, and Other Losses	647	838	1,471	2,266
Ending Balance	4,968	4,194	4,968	4,194
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	4,968	4,194	4,968	4,194
Loans and Leases Receivable, Gross	644,018		644,018		610,298
Total		604,307		604,307
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	644,018	604,307	644,018	604,307
Unallocated [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	5,922	0	5,926
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for Loan, Lease, and Other Losses	0	812	0	808
Ending Balance	0	6,734	0	6,734
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	$ 0	$ 6,734	$ 0	$ 6,734