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	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 55,251	$ 59,383
Unpaid Principal Balance	60,915	66,307
Related Allowance	9,461	8,824
Average Recorded Investment	49,794	78,175	87,979
Interest Income Recognized	625	679	369
Commercial, financial, agricultural and other with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	9,439	6,752
Unpaid Principal Balance	10,937	7,649
Average Recorded Investment	11,536	14,454	9,217
Interest Income Recognized	133	73	173
Real estate construction with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	236	3,486
Unpaid Principal Balance	476	6,664
Average Recorded Investment	1,190	5,923	11,912
Interest Income Recognized	19	47	0
Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	10,773	9,333
Unpaid Principal Balance	12,470	9,952
Average Recorded Investment	11,592	9,280	8,114
Interest Income Recognized	210	211	72
Commercial real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	8,768	13,606
Unpaid Principal Balance	10,178	14,719
Average Recorded Investment	8,830	27,881	28,574
Interest Income Recognized	98	250	66
Loans to individuals with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	288	289
Unpaid Principal Balance	337	307
Average Recorded Investment	308	255	103
Interest Income Recognized	4	3	2
Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	29,504	33,466
Unpaid Principal Balance	34,398	39,291
Average Recorded Investment	33,456	57,793	57,920
Interest Income Recognized	464	584	313
Commercial, financial, agricultural and other With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	24,826	21,482
Unpaid Principal Balance	25,583	22,082
Related Allowance	9,304	7,364
Average Recorded Investment	15,797	16,479	21,979
Interest Income Recognized	143	64	9
Real estate construction with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	414
Unpaid Principal Balance	0	737
Related Allowance	0	94
Average Recorded Investment	0	515	1,457
Interest Income Recognized	0	0	0
Residential real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	367	3,533
Unpaid Principal Balance	380	3,585
Related Allowance	56	1,282
Average Recorded Investment	357	3,200	1,599
Interest Income Recognized	14	31	15
Commercial real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	554	488
Unpaid Principal Balance	554	612
Related Allowance	101	84
Average Recorded Investment	184	188	5,024
Interest Income Recognized	4	0	32
Loans to individuals With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Subtotal With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	25,747	25,917
Unpaid Principal Balance	26,517	27,016
Related Allowance	9,461	8,824
Average Recorded Investment	16,338	20,382	30,059
Interest Income Recognized	$ 161	$ 95	$ 56

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

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Amount of allowance for credit losses related to recorded investment.

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