Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Recorded investment	$ 78,281		$ 107,555
Unpaid principal balance	109,125		139,687
Related allowance	12,689		17,778
Average recorded investment	96,859	82,156
Interest Income Recognized	161	65
Commercial, financial, agricultural and other with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	8,649		8,080
Unpaid principal balance	9,601		8,983
Related allowance	0		0
Average recorded investment	8,592	3,258
Interest Income Recognized	51	18
Real estate construction with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	6,122		8,491
Unpaid principal balance	27,005		35,555
Related allowance	0		0
Average recorded investment	7,877	3,646
Interest Income Recognized	0	0
Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	8,666		7,928
Unpaid principal balance	9,269		8,401
Related allowance	0		0
Average recorded investment	8,516	3,354
Interest Income Recognized	37	5
Commercial real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	16,711		33,259
Unpaid principal balance	23,250		35,401
Related allowance	0		0
Average recorded investment	33,978	24,275
Interest Income Recognized	11	23
Loans to individuals with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	240		256
Unpaid principal balance	247		256
Related allowance	0		0
Average recorded investment	247	0
Interest Income Recognized	1	0
Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	40,388		58,014
Unpaid principal balance	69,372		88,596
Related allowance	0		0
Average recorded investment	59,210	34,533
Interest Income Recognized	100	46
Commercial, financial, agricultural and other with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	25,702		26,532
Unpaid principal balance	26,655		27,412
Related allowance	9,846		10,331
Average recorded investment	25,803	30,350
Interest Income Recognized	19	3
Real estate construction with related allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	2,860		2,756
Unpaid principal balance	3,169		3,087
Related allowance	503		300
Average recorded investment	2,685	10,432
Interest Income Recognized	13	0
Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	2,663		2,695
Unpaid principal balance	2,666		2,696
Related allowance	1,091		780
Average recorded investment	2,457	952
Interest Income Recognized	4	7
Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	6,668		17,558
Unpaid principal balance	7,263		17,896
Related allowance	1,249		6,367
Average recorded investment	6,704	5,889
Interest Income Recognized	25	9
Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	0		0
Unpaid principal balance	0		0
Related allowance	0		0
Average recorded investment	0	0
Interest Income Recognized	0	0
Subtotal With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment	37,893		49,541
Unpaid principal balance	39,753		51,091
Related allowance	12,689		17,778
Average recorded investment	37,649	47,623
Interest Income Recognized	$ 61	$ 19